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Adults and larvae feed on the edges of leaves and may quickly defoliate young plants. They eventually strip all leaves from the haulm; exceptionally, tubers exposed at the soil surface are also eaten. Characteristic black and sticky excrement is left on the stem and leaves by the larvae and adults.
Delayed colonization of the crop reduces the time available for second and subsequent generations to develop (Voss et al., 1988). Delayed colonization could be achieved by various means, such as crop rotation, manipulation by planting date, setting up different barriers (such as plastic line trenches, portable trench barriers, mulching), and mechanical collection of beetles on overwintering sites or immediately after colonization.
Crop rotation delays colonization of the crop by overwintered adults and reduces the size of the population that subsequently develops within the crop. Population reductions of 90% or more have been reported in potato (Lashomb and Ng, 1984; Wright, 1984). The effect of crop rotation is increased with increased distance from the source of overwintering beetles. Follett et al. (1996) suggested 0.5 km as the minimum distance necessary to fully benefit from field rotation. Because rotated potato fields require fewer insecticides to control L. decemlineata (Speese and Sterrett, 1998), crop rotation is an important tool in delaying the development of resistance to insecticides (Roush et al., 1990). Saxon and Wyman (2005), in their suggestion of developing of an area-wide L. decemlineata pest management strategy, reported that long-distance rotations of more than 400 m were an effective cultural control management strategy to limit adult beetle infestations in the spring. This strategy can be optimized when collaborating growers are able to maximize their rotational distances by coordinating their rotational schemes over large areas. Deploying long-distance rotations over a large area over many years would limit L. decemlineata populations and could result in a signifcantly reduced L. decemlineata populations entering fields in the spring.
The planting date of a potato crop can be manipulated to reduce the population of second generation larvae produced in the crop. Early planting of short season varieties allows the crop to mature before the second larval generation is produced. In contrast, colonization of late planted, rotated potato plantings occurs later in the season causing most summer generation adults to emerge after the critical photoperiod for diapause induction has been reached. Consequently, these adults do not produce a second generation of larvae (Weber and Ferro, 1993).
Plastic-lined trenches, which serve as pitfall traps, and trap crops of early-planted potato have both been shown to effectively intercept overwintered adults in the spring, before they colonize the potato crop. Plastic-lined trenches that were V-shaped with an average width at the top of 740±7 mm and depth of 223±2 mm retained 95% of the beetles they trapped in controlled field experiments (Misener et al., 1993; Boiteau and Osborn, 1999). Because 50-75% of overwintered beetles disperse into a nearby potato crop by walking, properly designed trenches, positioned to intercept the dispersing beetles can provide a significant level of crop protection. At one location in New York, USA, more than 100,000 overwintered beetles were trapped in 91 m of trench (Moyer, 1993).
Overwintering beetles are frequently found in windbreaks and hedgerows adjacent to the crop at densities of hundreds per square metre, whereas densities within the fields average only 3-7 beetles per square metre (Weber and Ferro 1993, Hunt and Tan 2000). This distribution of overwintering beetles clearly has important implications for the management of L. decemlineata, particularly when another solanaceous crop is planned for the same or an adjacent field in the following year. For some growers crop rotation is not a viable option due to a shortage of land. Hunt and Vernon (2001) suggested that barriers placed along the margins of tomato and potato fields adjacent to preferred overwintering sites may be effective in preventing or slowing the entry of beetles into the crop in the spring. They designed an above-ground trench composed of an extruded, UV retarded PVC plastic trough, designed to allow L. decemlineata and other pests to enter the device and become trapped and killed inside. The above-ground trench can capture thousands of newly emerged L. decemlineata as they walk from their overwintering habitat into an adjacent crop, and also reduce crop damage. This trap is easily installed in the spring and removed in the fall, and is reusable for several years. The trench is black to raise the temperature, which results in an increase in beetle mortality, and is designed to release water and beneficial insects.
Mulching potato and aubergine plantings with straw has been shown to significantly reduce L. decemlineata infestations and defoliation, reduce the number of insecticide treatments needed and increase yield (Zehnder and Hough-Goldstein, 1990; Stoner, 1993; 1997). The mulch delays colonization and increases predation on eggs and larvae (Ng and Lashomb, 1983; Brust, 1994). Lower potato beetle populations and less damage to tomato has been reported in reduced tillage plantings than in conventional tilled plantings (Hunt, 1998).
Mechanical collection, use of propane flamers, use of pneumatic thermal machines or bio-collectors can prevent adult colonization and reduce larval damage (Boiteau et al., 1992; Karalus, 1994; Pelletier et al., 1995; Lague et al., 1999a, b; Derafshi, 2006). The bio-collector is a novel control method approved for use in organic potato cultivation in Germany. The collector, which is attached to a tractor, blows chrysomelids off the potato plants and collects them in trays. Collections are made two to three times per year depending on the level of infestation (Karalus, 1994). However, some authors stated that mechanical control causes undesirable damage and its efficacy should be improved (Sablon et al., 2013).
Although Boiteau et al. (2012) found in laboratory trials that wood ash is toxic to adult and larval stages of the L. decemlineata, the significant control observed in the laboratory did not extend to field application.
The chemical ecology of L. decemlineata is not yet completely understood and this incomplete knowledge makes semiochemical-based approaches inefficient when compared to traditional insecticide treatments (Sablon et al., 2013). According to Sablon et al. (2013a). Nevertheless, alternative strategies have potential to control L. decemlineata populations and include: (1) disorientation of L. decemlineata adults by masking potato volatile organic compounds (VOC) with intercropping cultures; (2) use of synthetic mixtures of volatiles and/or aggregation pheromone to trap beetles; (3) antifeedant sprays on potatoes; (4) increase, with genetic manipulations, the natural capacity of the plant to recognize the presence of L. decemlineata through chemical signals, thereby triggering defence mechanisms.
(1) Intercropping or companion planting may represent an efficient method to repel and/or confuse L. decemlineata foraging for host plants. Matthews et al. (1982) reported that the use of tansy as an intercrop in potato fields may result with 60-100% of decrease in the number of beetles present in the fields. The initial results reported by Thierry and Visser (1986; 1987) have shown that potato VOCs mixed with VOCs from tomato or cabbage disrupted searching behaviour of L. decemlineata females for host plants. Alghali et al. (2000) reported that L. decemlineata larvae were more frequently encountered in the pure potato plots compared with the potato/cabbage intercropped crops (either alternate rows of potato and cabbage or alternate row strips of potato and cabbage). Moreau et al. (2006) tested bush beans (Phaseolus vilgaris cv. Provider), flax (cv. Natasia), French marigold (Tagetes patula cv. Bolero), horseradish and tansy (Tanacetum vulgare) as companion plants to potatoes. In addition, some botanical and microbiological preparations (capsicain extract, garlic extract, neem extract, Bt product, adjuvant and pine extract) were evaluated. The results showed that companion planting and garlic and capsicain extracts did not reduce the densities of L. decemlineata on potatoes.
(2) The basic strategy of trap crops is aimed to attract L. decemlineata in a specific part of the field where potato plants are planted earlier or where potato plants are treated with semiochemicals. The next step is to treat only this part of field to eliminate attracted beetles.
In a small plot study in Massachusetts, USA, the colonization of potato fields by adults that overwintered in wooded borders of potato fields was reduced by about 60% when beetles in the trap crop were collected daily or killed with an insecticide (Weber et al., 1994). Trap crops have been shown to reduce L. decemlineata infestations and increase yields in tomato by 61-87% (Hunt and Whitfield, 1996).
L. decemlineata as a chewing insect is very sensitive to VOCs released by host plants, because the damage they induce in plant tissues increases the release of these compounds (Szendrei and Rodriguez Saona, 2010). Dickens (2000) investigated in laboratory trials behavioural responses of the L. decemlineata to volatiles emitted from solanaceous host plants, non-host legume plants and 13 synthetic blends of three individual chemicals emitted by potato plants. He found that L. decemlineata is attracted to blends of specific chemicals emitted by their host plants. In further investigations, Dickens (2002) reported a synthetic blend of three compounds ((Z)-3 hexenyl actetate. ±-linalool and methyl salicylate) which attracted second and fourth larval instars. This blend attracted newly emerged and 5-day-old adults in greenhouse trials (Martel et al., 2005a), and initial field tests confirmed the effect of the attractant blend and its usefulness for semiochemical-based control (Martel et al., 2005b). Pitfall traps baited with the attractant captured a greater number of L. decemlineata in potato fields. Moreover, fewer egg masses and larvae were observed in neighbouring untreated crops. The use of trap crops with the attractive blend allowed for a 44% pesticide reduction for similar yields compared to conventional methods (Martel et al., 2005b). In another study (Martel et al., 2007), this blend was coupled with pyrethroid insecticide at different concentrations. The use of this attracticide showed the same control efficiency as the commercial insecticide whilst using 92% less insecticide. Since many L. decemlineata populations have developed the resistance to permethrin, the authors suggested imidacloprid as a replacement.
A male-produced aggregation pheromone for L. decemlineata was identified as (S)-3, 7-dimethyl-2-oxo-oct-6-ene-1, 3-diol [(S)- L. decemlineata I] by Dickens et al. (2002). The pheromone has been shown to be attractive to L. decemlineata larvae (Hammock et al., 2007) as well as male and female adults (Dickens et al., 2002). Kuhar et al. (2006) demonstrated the attractiveness of (S)- L. decemlineata I in the field as well as its potential for integrated pest management of L. decemlineata. However, the synthetic routes for (S)- L. decemlineata I of high purity have been complicated and expensive. Therefore Kuhar et al. (2012) conducted pitfall trap studies to assess the relative attraction of L. decemlineata adults to synthetic mixtures of the (S) and (R) enentiomers of the pheromone. The results indicated that any further research as well as IPM strategies that incorporate L. decemlineata I as an aggregation pheromone should utilize blends containing more than 87% optical purity of the (S)-enentiomer of the pheromone. The main difficulty with semiochemicals is ensuring a controlled release during a long period (Sablon et al., 2013). The choice of dispenser is very important to control the release and to prevent molecule degradation (Heuskin et al., 2011).
(3) The basic concept of the use of antifeedants is to spray them on potatoes to deter L. decemlineata feeding and reduce damage of potato foliage. Many of antifeedants are constituent of plants from sagebrush community, hydroxides which also act as fungicides, alchocol extracts of the leaves and bark of Quercus alba L., limonin, sesquiterpenes (Sablon et al., 2013), α-mangostin (Kim and Lan, 2011), terpenoid lactones (Szczepanik et al., 2005) and extracts from various plants (Gokce et al., 2006a, Pavela et al., 2004, 2009) including potatoes (Szafranek et al., 2008). Neem extract, a well-recognized botanical insecticide, also shows antifeeding activity against L. decemlineata, but the magnitude of the effects depends on the dominant L. decemlineata life stage present when application was made and on attack intensity (Zehnder and Warthen, 1988; Bezjak et al., 2006; Igrc Barcic et al., 2006). Such molecules and blends of chemicals not only reduce feeding but also may deter oviposition by females as shown for saponins (Waligora-Rosada 2010) and citrus limonoids (Murray et al., 1995). Very often antifeedants are considered as synthetic insecticides whereas they originate from plants and should therefore be considered as botanical insecticides. According to Sablon et al. (2013a) this is one of the reasons why the commercialization of antifeedants has been generally unsuccessful.
Edovum puttleri has been released in several areas in the eastern USA and is being investigated for possible release in Europe (Schauff, 1991). Research on E. puttleri in relation to its potential for biological control of the Colorado potato beetle in the USA is reviewed by Schroder and Athanas (1989a, b) and Schroder et al. (1985).
The ability of augmentative releases of E. puttleri to control Colorado potato beetle was assessed on aubergine in New Jersey, USA (Lashomb et al., 1987) and Italy (Pucci and Dominici, 1988) and on potato in Maryland, USA (Schroder and Athanas, 1989a, b) and Ontario and New Brunswick, Canada (Sears and Boiteau, 1989). The most favourable results were obtained for aubergine in New Jersey (Lashomb et al., 1987) and for potato in Maryland (Schroder et al., 1985; Schroder and Athanas, 1989a, b). However, detailed comparisons between these studies were limited by the variation in numbers of wasps released and the methods used to evaluate the impact on the host populations (Van Driesche et al., 1991). Results reported by Van Driesche et al. (1991) for augmentative releases of E. puttleri against L. decemlineata on potatoes in Massachusetts, USA, confirmed the findings of Sears and Boiteau (1989) that releases of E. puttleri are ineffective against first-generation eggs. Parasitoid releases had greater impact on second generation eggs. The release of 47,000 wasps in 1988 against the second generation, without pesticide, resulted in 34.4% parasitism and 16.1% host feeding; a total reduction of 50.5%.
Releases of Podisus maculiventris, at 76,000/ha, reduced egg numbers of Colorado potato beetle in the second and third generations in field model experiments in Moldova, Russia (Novozhilov et al., 1991). The complex of natural enemies, including 26 carabids, 11 coccinellids and three chrysopids, added to the beneficial effect of the pentatomid. When L. decemlineata eggs were exposed to P. maculiventris and twelve-spotted ladybeetle (Coleomegilla maculata), predation on L. decemlineata eggs by both predators together did not increase significantly over levels inflicted by either predator alone (Mallampalli et al., 2002). Aldrich and Cantelo (1999) demonstrated that pheromones of P. maculiventris could be used to attract females and thereby enhance populations of the predator within a potato planting.
Augmentative releases of Perillus bioculatus controlled first-generation eggs and larvae of L. decemlineata in field-plot tests conducted during two seasons under short-season conditions in Quebec, Canada (Cloutier and Bauduin, 1995).
Sablon et al. (2011) reported on the results of laboratory assay in which the use of lacewing larva Chysoperla carnea allowed an efficient reduction of L. decemlineata eggs and larvae. Sablon et al. (2013b) have shown that the first and second instar of C. carnea only consumed eggs, whereas the third instar of consumed all L. decemlineata immature stages. The third instar killed four times as many L. decemlineata larvae than other larval stages. However, field assays are needed to confirm the efficiency of this natural enemy under field conditions.
The nematode Pristionchus uniformis reduced populations of L. decemlineata in greenhouse and field experiments in Poland (Fedorko and Stanuszek, 1971). Variable suppression of L. decemlineata populations was achieved in New Brunswick and Prince Edward Island, Canada, following application of the entomopathogenic nematode Steinernema carpocapsae (Stewart et al., 1998). Among the four investigated nematode species (Steinernema feltiae, S. carpocapsae, Heterorhabditis bacteriophora and H. megidis), Trdan et al. (2009) found the application of higher concentrations of S. feltiae to be the best for the control of overwintering adults for the purpose of preventing mass appearance. Adel and Hussein (2010) reported a significant reduction of the number of damaged leaves and lower index of damage achieved with Steinernema feltiae and Heterorhabditis bacteriophora foliage application. Ebrahimi et al. (2011) investigated the effect of the sublethal concentrations of the same nematode species on L. decemlineata. They found that sublethal nematode concentrations caused wing deformation and delayed metamorphosis which may affect L. decemlineata adult fitness and have long term effect on population pressure.
Most work on fungi for the biological control of chrysomelid hosts has focused on the use of Beauveria bassiana against the Colorado potato beetle. Attempts at controlling L. decemlineata with Beauveria in Russia and Eastern Europe began with large-scale research programmes in Russia that led to the mass production and relatively widespread use of Beauveria-based products, generally bearing the name 'boverin' (Ayleshina, 1978; Lipa, 1985). This Beauveria-based approach to the control of the Colorado potato beetle was most effective on low and moderate populations of the beetles. The extensive use of B. bassiana against L. decemlineata in biological control programmes in Russia during the 1970s and 1980s is summarized by Feng et al. (1994).
During the mid-1980s, a pilot test programme studying the efficacy of B. bassiana for the biocontrol of Colorado potato beetle on potatoes in the USA showed mixed results, but demonstrated that, at its best, the fungus could provide foliar protection and result in yields approaching those of pesticide-treated control plots (Watt and LeBrun, 1984; Hajek et al., 1987). In America, the use of B. bassiana to control L. decemlineata never achieved either the scale or the reported success of the Soviet effort and was eventually abandoned by the commercial firm that provided the formulated fungus for the pilot test programme.
Kryukov et al. (2014) established for the first time that the peroral effect of the fungal culture of Cordyceps militaris resulted in a dose-dependent decrease in survival, and delayed developmental time and moulting, as well as increased sensitivity of larvae to the fungus Beauveria bassiana.
Following the invasion of the potato-growing areas of Continental Europe, attempts were made to introduce North American natural enemies, known to be important mortality factors, to France, from 1929-1940. Two tachinids, Myiopharus doryphorae and M. aberrans; two pentatomids, Perillus bioculatus and Podisus maculiventris; and a carabid, Lebia grandis, were cultured and released, but none became established. Work resumed in 1957 under a co-operative programme involving 12 countries. Efforts concentrated on attempting to establish P. bioculatus, again without success; it was concluded that P. bioculatus was not adapted to conditions in Europe. Further introductions of M. doryphorae were also made in France but the European strain of L. decemlineata was resistant to parasitism by this tachinid (Greathead, 1976). Attempts at introducing natural enemies were then abandoned until the discovery of Endovum puttleri, which was imported into several countries and released but did not persist for long periods. Recent efforts in the Czech Republic to use Beauveria to control L. decemlineata have shown some success (Dirlbekova and Dirlbek, 1987; Dirlbek and Dirlbekova, 1987; Dirlbek, 1988; Dirlbekova et al., 1992).
Effective control of L. decemlineata was achieved in potato fields in New Jersey, USA, in 1989-1990, using foliar applications of Bacillus thuringiensis subsp. tenebrionis. The bacillus was sprayed when an average 1-30% of marked egg masses had hatched. Significantly greater defoliation, greater numbers of third- and fourth-instar larvae, and lower yields occurred in plots when the initial application was delayed until 6 days after 30% egg hatch, compared with plots treated at 30% egg hatch (Ghidiu and Zehnder, 1993). When applied properly, foliar applications of commercial formulations of B. thuringiensis subsp. tenebrionis provided effective control of L. decemlineata and protected the potato crop from defoliation. Because B. thuringiensis subsp. tenebrionis must be ingested to be effective, is most effective against small larvae, and has only limited residual activity once applied to foliage, proper timing of applications and thorough spray coverage of the crop are critical to effective control (Zehnder and Gelernter, 1989; Ferro and Lyon, 1991; Zehnder et al., 1992; Dubois and Jossi, 1993; Korol' et al., 1994; Ferro, 2000). Ghassemi-Kahrizeh and Aramideh (2014) have proven in laboratory assays that there is a remarkable synergistic effect of Henna powder on Bacillus thuringiensis efficiency.
In organic agriculture pest management, the mainstay for insect control, including L. decemlineata control, has been pyrethrum Chrysanthemum cinerariaefolium. This has a knock-down effect, but a short residual activity (Zehnder et al., 2007). Piperonil butoxide (PBO) is also used as a synergist for pyrethrins and the synthetic pyrethroid insecticides. Because pyrethrins are harmful to beneficial animals, such as predatory mites, parasitoids, and honey bees, the use of the synergist reduces the concentration of pyrethrins necessary for pest control. Owing to the concern over PBO, dillapiol is considered as a potential replacement. Dillapiol, which was first found in the Indian dill, Anethum sowa Roxb. ex Fleming (Apiaceae), is nearly as effective as PBO as a pyrethrin synergist (Joffe et al. 2012). In laboratory trials that pyrethrum efficacy was increased 2.2 times with the SS strain and 9.1 times with the RS strains of L. decemlineata by using pyrethrum + diallipol (Liu et al., 2014). In field trials with the pyrethrum+ diallipol formulation demonstrated efficacy ≥10 times than pyrethrum alone.
Besides pyrethrum extracts, many plant extracts and essential oils have been tested against L. decemlineata. Among the tested plants, Humulus lupulus extract has been proven by Gokce et al. (2006b) and by Cam et al. (2012) to be toxic to L. decemlineata larvae. The essential oils of Mentha longifolia (Rafiee-Dastjerdi et al., 2014), Letharia vulpina and Peltigera rufescens (Emsen et al., 2013) and Piper nigrum (Scott et al., 2003) can also control L. decemlineata.
Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers. Several studies have shown that some of the monoterpens have potential for L. decemlineata control. Monoterpene hydrocarbons exhibited high toxicity as compared with oxygenated monoterpenes; 1,8-cineole, fenchone, β-pinene and γ-terpinene can be used as potential control agents against both L. decemlineata larvae and adults (Kordali et al., 2007). Khorram et al. (2011) tested 6 pure monoterpene hydrocarbons as a single compounds or mixtures; all six tested compounds (monoterpene hydrocarbons) can be used as potential control agents against both larvae and adults of L. decemlineata, either as single compounds or in mixtures. Mahdi et al. (2011) tested 12 pure oxygenated monoterpenes as two different doses for their toxicity against second and third instar of larvae and adults of L. decemlineata; out of 12 tested, oxygenated monoterpenes, fenchone, linalool, citronella and menthone showed a strong toxicity against the tested developmental stages.
Transgenic potatoes expressing the gene for B. thuringiensis subsp. tenebrionis Cry3A delta -endotoxin were approved for commercial use in the USA in 1995. In 1998, transgenic potato varieties expressing the Cry3A toxin were planted on approximately 20,000 hectares in the USA. Although these transgenic potato varieties are highly toxic to Colorado potato beetle (Perlak et al., 1993; Wierenga et al., 1996) and provide excellent control of the beetle, planting of Cry3A toxin-expressing transgenic potato varieties declined dramatically by 2000 due to concern over consumer resistance to purchasing transgenic potatoes and products made from them. An additional factor contributing to this decline was competition from new, conventional insecticides that controlled a broader spectrum of potato pests. The future role of transgenic potato varieties in the Colorado potato beetle management is currently uncertain, despite their effectiveness and considerable evidence that they have no significant effect on populations of natural enemies (Hoy et al., 1998; Riddick et al., 1998). Transgenic lines of aubergine expressing the Cry3A endotoxin (Hamilton et al., 1997b) and lines expressing the Cry3B endotoxin (Arpaia et al., 1997; Mennella et al., 1998), although not commercially available, have been shown to provide control of Colorado potato beetle.
Because of the potential that Colorado potato beetle will develop resistance to the Cry3A endotoxin if these transgenic potato varieties are widely planted, significant effort has been directed towards the development of resistance management strategies for transgenic potatoes. Proposed resistance management strategies for potato focus on using varieties that express a high dose of the toxin in conjunction with plantings of refuge areas of non-transgenic potato or other hosts in which beetles are not controlled. The level of toxin expressed should be high enough to kill any individuals that are heterozygous for resistance alleles. The size and location of refuge plantings must be such that any homozygous resistant beetles selected on the transgenic crop mate with homozygous susceptible beetles produced in the refuge. The progeny of such matings would be heterozygous for the resistance allele and would be killed if they fed on the transgenic crop. Strategies for managing resistance to transgenic potatoes are discussed by Gould et al. (1994) and Hoy (1999). Ochoa-Campuzano et al. (2013) identified prohibitin, an essential protein for L. decemlineata larval viability. They explored the possibility for prohiobitin-1 silencing in L. decemlineata larvae in order to reach higher efficacy of Cry3Aa toxin. Cooper et al. (2006) assessed the effectiveness of the protein avidin against the Colorado potato beetle neonates in a no-choice detached leaf bioassay at 0, 17, 34, 51, 102, and 204 μg avidin/ml over 12 d. The combined effects of avidin (136 μg avidin/ml) with Bt-Cry3A or leptines were evaluated with neonates and third instars over 12 and 6 days, respectively. Survival of third instars on the Bt-Cry3A with avidin was significantly reduced after 3 days compared with survival on the Bt-Cry3A, suggesting the addition of avidin may increase susceptibility to Bt-Cry3A.
The Colorado potato beetle spiroplasma (CPBS) appears to be host (genus)-specific and is transmitted among larvae and adults during regurgitation and defecation. However, researchers have adopted a strategy to engineer spiroplasma with an insect-lethal gene because the spiroplasma appears to be a commensal (Hackett et al., 1988; Gasparich et al., 1993a, b). The delta-endotoxin gene for B. thuringiensis subsp. tenebrionis (Btt) may be an appropriate gene for this purpose because the Colorado potato beetle is susceptible to the toxin and the CPBS adheres to the midgut microvilli (Hackett and Clark, 1989), which is the site of action of the delta-endotoxin. This system would allow multiplication and spread of the genetically-engineered microorganism throughout the crop in contrast to direct treatment of beetles with the Btt beta-endotoxin.
Zhu et al. (2011) reported the results of study in which the potential of feeding dsRNA expressed in bacteria or synthesized in vitro to manage populations of L. decemlineata was investigated. Feeding RNA interference (RNAi) successfully triggered the silencing of all five target genes tested and caused significant mortality and reduced body weight gain in the treated beetles. This study provides the first example of an effective RNAi response in insects after feeding dsRNA produced in bacteria. The obtained results suggest that the efficient induction of RNAi using bacteria to deliver dsRNA is a possible method for management of L. decemlineata. This method is still in the experimental stage.
The incorporation of varietal resistance to Colorado potato beetle has emphasised the transfer of resistance traits to S. tuberosum from other Solanum species using a variety of techniques to obtain successful interspecific crosses (Shapiro et al., 1991; Tingey and Yencho, 1994). Emphasis has been placed on resistance derived from Solanum berthaultii, which is mediated in large part by glandular trichomes on the foliage (Yencho et al., 1996) and on resistance derived from Solanum chacoense, which is mediated by high concentrations of leptine glycoalkaloids in the foliage (Sinden et al., 1986; Sanford et al., 1997; Yencho et al., 2000). Potato breeding lines with resistance to potato beetle have been released (Plaisted et al., 1992; Lorenzen and Balbyshev, 1997). Other potentially valuable mechanisms of resistance have been identified as well (for example, Balbyshev and Lorenzen, 1997).
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The Colorado beetle, L. decemlineata, is one of the most economically damaging insect pests of potato in the many countries where it now occurs (Hare, 1990). L. decemlineata adults and larvae indirectly reduce potato tuber yields by devouring foliage. If plants become entirely defoliated prior to tuber initiation, total crop loss will result. L. decemlineata will also attack tomato and aubergine. In many areas, it is the only pest of ware potato crops against which insecticides have to be applied.
L. decemlineata originated in southwestern North America where it utilized a variety of solanaceous weed species (Hsiao, 1981). In the first half of the 19th century, its host range was expanded to include potato, which was grown east of the Rocky Mountains as the western USA became settled. By the late 19th century, the distribution of L. decemlineata continued eastward throughout North America and by the early 20th century it had eventually spread into Europe and Asia (Hsiao, 1981). L. decemlineata is currently distributed between latitudes 15° and 60°N. It does not occur in general in tropical countries, nor in most of eastern Asia, Korea, Japan, India, northern Africa, or the temperate Southern Hemisphere (Vlasova, 1978; Worner, 1988; Jolivet, 1991).
Perhaps the greatest economic impact that L. decemlineata has had on agriculture has been since its development of resistance to insecticides. L. decemlineata has become resistant to >25 insecticides belonging to the traditional chemical classes (Forgash, 1981, 1985; Gauthier et al., 1981; Heim et al., 1990; Roush et al., 1990; Tisler and Zehnder, 1990; Bishop and Grafius, 1991; Georgiou and Lagunes-Tejeda, 1991). Factors responsible for insecticide resistance development in this pest are described in detail in Zehnder et al. (1994). In the late 1980s and early 1990s, potato growers in the eastern USA used as many as 12 insecticide applications averaging $145 per ha per season to manage L. decemlineata (Zehnder and Evanylo, 1989; Zehnder et al., 1995). Currently, the cost of controlling L. decemlineata infestations in the eastern USA averages between $138 and 368 per ha (Grafius, 1997). Because insecticide resistance in L. decemlineata populations is inevitable, agribusiness industries continue to invest millions of dollars into developing new insecticides and genetically modified crops that produce insecticidal toxins.
Crop yield and financial losses attributed to L. decemlineata are not frequently published nor are they discussed in major reviews of L. decemlineata biology and management (Lashomb and Casagrande, 1981; Radcliffe, 1982; Ferro and Voss, 1985; Hare, 1990; Zehnder et al., 1994). This may in part be due to the lack of controlled, replicated experiments in commercial fields required to document such information. Further, crops are often attacked by multiple pests and the contribution of yield loss due to a single pest species is often impossible to determine. One of the few examples in which losses due to L. decemlineata have been documented was in Michigan (Grafius, 1997). In 1994, Michigan potato growers spent $6.8 million to control L. decemlineata and still experienced losses in tuber yield totalling $7.0 million, which together was nearly 14% of the overall crop value. With the exception of Grafius 1997 study, there is a paucity of quantitative crop loss and financial loss information. Therefore, the remaining review of L. decemlineata economic impact on its hosts will include major findings that have advanced our knowledge in (1) predicting crop yield loss due to feeding injury and (2) developing economic injury levels and thresholds for use in managing this pest.
Economic injury levels and economic thresholds are major components in the decision-making process of pest management (Pedigo and Higley, 1992). Developing economic injury levels and thresholds requires knowledge of the market value of the crop, the cost of managing the pest and the crop yield response to pest density or damage. Since the late 1970s, considerable effort has been made in identifying tuber yield responses of potato to defoliation and density of L. decemlineata adults and larvae. Fewer studies have examined the yield/damage relationship in aubergine and tomato.
Crop Response to Defoliation by L. decemlineata
The response of solanaceous crops to defoliation by L. decemlineata varies considerably with the phenological stage of the plant. For example, tomato seedlings cannot recover from extensive feeding by L. decemlineata adults, but as plant canopy increases, the level of defoliation that can be tolerated also increases (Schalk and Stoner, 1979). Potato has been shown to be least susceptible to yield loss when defoliated very early or late in the season (Hare, 1980; Ferro et al., 1983). Hare (1980) and Zehnder and Evanylo (1989) demonstrated that potato could withstand high levels of defoliation within a few weeks before harvest. Many studies have shown that potato plants are least tolerant of defoliation during the bloom stage (Cranshaw and Radcliffe, 1980; Hare, 1980; Wellik et al., 1981; Ferro et al., 1983; Shields and Wyman, 1984; Dripps and Smilowitz, 1989; Senanayake and Holliday, 1990; Nault and Kennedy, 1996). The only exceptions were reported by Zehnder and Evanylo (1989) and Zehnder et al. (1995) who showed that potato was most sensitive to yield loss when defoliated during prebloom.
Igrc et al. (1999) stated that the level of yield loss provoked by L. decemlineata defoliation depends upon the correspondence between severe leaf damage and tuberization process. In multiple year field trials they reported yield increase on treated plots from 18% up to 817% in the year when complete defoliation occurred at the beginning of tuberization (i.e. pre-bloom). The initiation of tubers is the key developmental occurrence in the life of potato crop. If the leaf damage occurs in the early stage of tuberization, the yield loss can be very high and vice versa. In most cultivars tuberization corresponds with floral initiation. Significant variation in yield losses caused by L. decemlineata in field trials among years was also observed by Igrc Barcic et al. (2006).
Several approaches have been taken to describe potato response to defoliation in order to predict tuber yield losses. Some have modelled potato growth over an entire season in which defoliation was either continuous or occurred during a specific growth period (Logan and Casagrande, 1980; Logan, 1981; Elkington et al., 1985; Dripps and Smilowitz, 1989; Johnson et al., 1996). Others have described the relationship between potato tuber yield and defoliation when defoliation occurred during a specific plant growth stage (Nault and Kennedy, 1996; Nault and Kennedy, 1998). Cotty and Lashomb (1982) determined the relationship between yield of aubergine fruit with densities of L. decemlineata, whereas Cantelo and Cantwell (1983) described the response of tomato to simulated L. decemlineata feeding.
Development of Economic Injury Levels and Thresholds
Economic injury levels and thresholds have been developed for optimizing the use of insecticide applications to control L. decemlineata infestations. Economic injury levels and thresholds have been based on levels of defoliation as well as densities of larvae and adults. Defoliation-based thresholds have been reported to vary depending on the phenological stage of the plant. Zehnder et al. (1995) developed defoliation-based thresholds of 20% during plant emergence to early bloom, 30% during early to late bloom, and 60% during late bloom to harvest. Defoliation-based thresholds also have been reported to vary within a phenological stage. For example, Zehnder et al. (1995) recommended a threshold during the bloom stage as high as 30% for the cultivar Superior in Virginia, whereas Shields and Wyman (1984) recommended a threshold of only 10% for Superior and Russet Burbank in Wisconsin (Shields and Wyman, 1984). In North Carolina, the level of defoliation that Atlantic potato was shown to tolerate during bloom varied depending on the statistical approach used to develop the threshold (Nault and Kennedy, 1998). The level of defoliation deemed tolerable by potato was usually much higher using mean separation analysis than using regression. For example, the same data set analyzed using mean separation analysis indicated that potato could withstand 44% defoliation during bloom, whereas non-linear regression analysis indicated that potato could only tolerate 13% defoliation during bloom. Both types of analyses have limitations (see Nault and Kennedy, 1998).
The relationship between potato yield and insect density also has been modelled to develop economic injury levels (Hare and Moore, 1988; Senanayake and Holliday, 1990; Mailloux et al., 1991, 1995). Based on this approach, economic injury levels of 5.8 overwintered adults and 10 summer-generation adults per plant have been reported (Mailloux et al., 1995). For larvae, economic injury levels have been reported to be as high as 12 per stalk (Mailloux et al., 1991) and as low as 0.14 to 0.82 per plant (Senanayake and Holliday, 1990). In processing tomato cultivars in Maryland, yield loss did not result until the number of L. decemlineata adults and larvae exceeded 1.25 per plant and 2 per plant, respectively (Linduska and Dively, 1990).
Use of Economic Thresholds for Managing L. decemlineata
There are few studies in which the benefits of using economic thresholds for managing L. decemlineata have been reported. In several instances, however, potato tuber yield did not significantly differ between an economic-threshold-based management approach and the conventional management program. Yet, fewer insecticide applications were needed to manage L. decemlineata infestations when an action threshold was used (Wright et al., 1987; Stewart and Dornan, 1990; Zehnder et al., 1995).
Despite the success of using action thresholds to optimize the use of insecticides for managing L. decemlineata, preventative control approaches have been commonly used in the USA. For example, in-furrow applications of imidacloprid and use of transgenic potato containing the Bacillus thuringiensis subsp. tenebrionis endotoxin have been used to provide near season-long and season-long protection of L. decemlineata, respectively. Similarly, because L. decemlineata adults may completely devour tomato as soon as plants emerge or immediately after seedlings are transplanted, a preventative approach using insecticides at planting or transplanting is taken to prevent loss (Ghidiu, 1984; Ghidiu and Oetting, 1987).