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Newly-hatched larvae feed primarily on root hairs. As the larvae grow and their food requirements increase, they burrow into roots. Larval damage is usually most severe after the secondary root system is well established and brace roots are developing. Root tips appear brown and often contain tunnels. In many cases, they are chewed back to the base of the plant. Larvae may be found tunnelling in larger roots and occasionally in the plant crown. Larvae may burrow through plants near the base, causing stunting or death of the growing point and frequently causing tillering. Root feeding commences shortly after plant emergence and early symptoms are expressed as drought or nutrient deficiencies. Plant lodging occurs later in plant development. Sites of larval damage are often pathways for infection by disease pathogens, resulting in root rots.
Adult beetles cause damage by feeding principally on pollen, silk and young kernels. Silk clipping near the husk during anthesis can cause reduced seed set in maize, which may only be observed at the time of harvest.
See Levine and Oloumi-Sadeghi (1991), Toepfer et al. (2005b), Gray et al. (2009), Ivezic et al. (2009), Narva et al. (2013) and Van Rozen and Ester (2010) for a review of management strategies for D. virgifera virgifera. Further information can also be found in a series of reviews in a special issue of Agricultural and Forest Entomology on D. virgifera virgifera research (Agricultural and Forest Entomology (2009), 11(1):3-60).
The capacity for natural spread of D. virgifera virgifera is such that it is difficult to propose measures for its prevention. European countries have put in place a monitoring network using pheromone traps to follow spread. In cases of new introductions, immediate insecticide treatments have to be carried out, no maize is allowed to be grown in a focus zone around the introduction point and crop rotation is obligatory in a safety zone around the focus zone (Byrne, 2003).
The main aim at present is to contain D. virgifera virgifera in Europe and to delay its impact as much as possible, mainly by applying crop rotation.
See European Commission decision 2003/766/EC (Byrne, 2003).
In 2014 the European Commission decided to withdraw the recognition of D. virgifera virgifera as a regulated harmful organism with quarantine status by deleting it from Annex 1 to Council Directive 2000/29/EC. The containment measures were also not regarded as successful and Decision 2003/766/EC was therefore repealed. EU Member states are now responsible for providing guidelines for D. virgifera virgifera management, especially taking the principles of IPM into account (Borg, 2014).
Crop rotation is an effective control method for D. virgifera virgifera as the eggs are mainly laid in maize, and the larvae must feed on maize roots to complete their development (Ostlie and Noetzel, 1987; Levine and Oloumi-Sadeghi, 1991). However, a 100% crop rotation would be too strict as an 80% rotation of maize can keep D. virgifera virgifera below the economic threshold. In some cases (e.g. Serbia) even a 60% rotation was sufficient for control. The economic threshold is usually reached when less than 40% of maize fields are rotated. Rotation of each field within 3 years can also lead to about 20% fewer fields that need to be rotated (Szalai et al., 2013).
In principle, all possible crops, fallows or vegetables can be rotated with maize for D. virgifera virgifera management (Kiss et al., 2005a). However, certain crops, such as soyabean or monocotyledonous crops, might be less promising in long-term rotation with Diabrotica-infested maize fields. Many Poaceae are known to serve to some degree as secondary food plants for D. virgifera virgifera larvae (Branson and Ortman, 1967, 1970; Moeser, 2003; Moeser and Hibbard, 2005) and adults feed on nearly every pollen source (Moeser and Hibbard, 2005). However, larval damage on cultivated poaceous plants other than maize has not yet been recorded. In the corn belt of the USA, where soyabean was rotated with maize regularly and over many years, an increased oviposition of D. virgifera virgifera into soyabean was observed and larvae developed in the maize planted in the following year ('crop rotation resistance phenotype') (Gray et al., 1998, 2009). This rotation resistance developed from a decreased fidelity to maize by D. virgifera virgifera females as a consequence of a limited defined crop rotation scheme (Levine et al., 2002). Damage to rotated maize was first reported in an east-central region of Illinois and continued to spread throughout the corn belt (Gray et al., 2009) but at a slow expansion rate (Dunbar and Gassmann, 2013). Furthermore, crop rotation may not be completely effective in 1-year rotations due to the presence of volunteer maize. D. virgifera virgifera has low survivorship <0.1%) over two winters of diapause, but a 1-year rotation may select for rootworms with extended diapause (Krysan et al., 1984).
Most studies have found no significant differences in D. virgifera virgifera oviposition among various tillage practices. However, no-till appears to have the lowest egg mortality.
Delayed planting may result in decreased root damage as eclosed larvae can only survive a few days without feeding on suitable hosts. If planting is delayed until early June, root damage is negligible and soil insecticide usage is not warranted (Musick et al., 1980). Later flowering maize can attract D. virgifera virgifera from surrounding infested maize fields. Thus, late-planted strips of maize can be used as trap crops, although this technique is not widely practised, as its effectiveness is inconsistent.
D. virgifera virgifera has few effective natural enemies in its area of origin in Central America (Eben and Barbercheck, 1996; Kuhlmann and van der Burgt, 1998). In North America, natural enemies undoubtedly help to reduce and stabilize rootworm populations but other control interventions are normally required to bring D. virgifera virgifera populations below an economic threshold. In Europe, host-specific and/or effective indigenous natural enemies do not attack any of the life stages of D. virgifera virgifera (Toepfer and Kuhlmann, 2004a).
Classical biological control
Classical biological control provides an opportunity to reconstruct the natural enemy complex of D. virgifera virgifera populations in Europe (Kuhlmann and Burgt, 1998) and partly in North America. The natural enemy complex of Diabrotica species was surveyed in their area of origin in Central America (Kuhlmann et al., 2005) and Celatoria compressa was the only parasitoid found on the target species, D. virgifera virgifera. Its host range is considered to be restricted to Diabroticite beetles, and thus C. compressa would be safe for introduction because direct and indirect impacts on other organisms would be extremely low (Kuhlmann et al., 2005).
Inundative biological control
Entomopathogenic nematodes have great potential as biological control agents of D. virgifera virgifera (Cabanillas et al., 2005). Several commercially available nematode species have proven effective at killing D. virgifera virgifera larvae; for details, see Cabanillas et al. (2005) and Toepfer et al. (2005a). The nematode species Heterorhabditis bacteriophora has been evaluated as a control agent across numerous studies on a field scale and could reach similar control efficacies to commercially available insecticide treatments (Toepfer, 2010a, b). The first large scale applications of nematodes as a commercial product for D. virgifera virgifera control was carried out in Austria in 2014 (product name: DIANEM ®).
The entomopathogenic fungi Beauveriabassiana and Metarhiziumanisopliae naturally attack D. virgifera virgifera (Toepfer and Kuhlmann, 2004a; Pilz, 2008; Rudeen et al., 2013). Diabrotica populations in the USA are not usually regulated by fungi (Maddox and Kinney, 1989). However, fungi-based products are registered against soil pests in several countries (Inglis et al., 2001) suggesting further research should be carried out towards the development of a fungal biocontrol product against D. virgifera virgifera (Toepfer et al., 2005b). The commercially available M. anisopliae strain BIPESCO5/F52 (product name: GranMet) is included in Annex 1 of Directive 91/414/EEC. Its efficacy against D. virgifera virgifera in the field is currently lower than with insecticides and nematodes (Pilz et al., 2009) but opens further options for biological control. The EU project 'InBioSoil' (www.inbiosoil.uni-goettingen.de) further explores the potential of BIPESCO5 through the use of fungal formulations for soil application and efficacy enhancing agents (e.g. semiochemicals).
Conventional breeding for resistance has resulted in germplasm with moderate levels of resistance to Diabrotica feeding (Knutson et al., 1999). There is currently no maize cultivar with native resistance available against D. virgifera virgifera, due to the limited success of conventional breeding strategies in the past (Moeser and Hibbard, 2005). The predominant mechanism for resistance is tolerance rather than antibiosis or antixenosis, for example, some maize cultivars demonstrate a tolerance to drought stress and larval feeding through the ability to regenerate roots (Branson et al., 1982). Hydroxamic acids have been identified as resistance factors to D. virgifera virgifera larvae in maize root tissue (Xie et al., 1990) and may occur in some commercial hybrids (Assabgui et al., 1993). Large screenings with a focus on root injury evaluation after artificial egg infestation identified resistance genotype but with no information on the underlying resistance mechanism. The use of more advanced genetic/genomic screening techniques (QTL, micorarrays and metabolite analysis) offers the potential to develop non transgenic maize hybrids with antibiosis against D. virgifera virgifera (Gray et al., 2009). The first steps in this process have been made (e.g. 'SUM' cultivars; El-Khishen et al., 2009) and research is ongoing, especially in Europe, to develop breeding programmes for native resistance (Ivezic et al., 2009).
Genetically modified resistance
Genetically modified maize varieties with Bt toxin expressed in their roots (Cry3Bb1, Cry34Ab1/ Cry35Ab1 and mCry3A) can avoid larval damage and have been commercialised in the USA since 2003 (Ward et al., 2005). Seven transgenic events targeting rootworms were released between 2003 and 2013 including pyramided traits with more than one Bt protein and different modes of action (first on the market SmartStax ® in 2009). More stacked Bt events were released in 2014 (eCry3.1Ab+mCry3A). The transgenic maize cultivars have been proven as highly effective (>95%) and were quickly adapted by farmers. Insect resistance management (IRM) plans were established to delay resistance to Bt maize by planting a refuge with a non Bt crop habitat.
In 2009, four populations of D. virgifera virgifera with evolved resistance to Cry3Bb1 were identified in Iowa (Gassmann et al., 2011, 2012; Devos et al., 2013). Resistance was probably triggered through the extensive use of the same Bt-maize repeatedly and exclusively (Devos et al., 2013). The ability of cross resistance to other single events (e.g., Cry34Ab1/ Cry35Ab1) is also possible (Devos et al., 2013). Caution should therefore be taken to mitigate any risks of resistance to other Bt events. Resistance models predict that resistance may be delayed with pyramided traits in Bt-maize in the absence of potential cross resistance (Onstad and Meinke, 2010). The use of these pyramided traits is increasing (Storer et al., 2012) but the Bt maize 'landscape' currently comprises a mixture of single and multiple toxins.
Growing of Bt maize in Europe is often denied by EU or national authorities (some countries have banned the use of Bt maize under the safeguard clause Articles 16 and 18) so there is currently no transformed Bt event for D. virgifera virgifera resistance authorised for cultivation (Meissle et al., 2011).
The use of RNAi (RNA interference), in which double-stranded RNA (dsRNA) molecules trigger gene silencing in a sequence specific manner, could provide a new option for D. virgifera virgifera control (Miyata et al., 2014). D. virgifera virgifera larvae are sensitive to an oral RNAi approach and transgenic plants expressing D. virgifera virgifera dsRNAs show a significant reduction in larval feeding damage (Baum et al., 2007). Further studies will evaluate its potential as a management option and the likelihood for commercial use (Burand and Hunter, 2013).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
D. virgifera virgifera together with D. barberi are the most serious insect pests of maize in the major maize-producing states of the north-central USA (Levine and Oloumi-Sadeghi, 1991). The larvae damage maize roots, which reduces the ability of the plant to absorb water and nutrients from the soil and causes harvesting difficulties due to plant lodging. Adult feeding on silks interferes with pollination. The quantification of yield loss varies according to cultivation practices and location of the field. Generally yield losses have been estimated at around 15% for every node (i.e. circle of roots) damaged from larval feeding (Tinsley et al., 2013).
The costs of soil insecticides to control larval damage to roots, and of aerial spray to reduce adult damage to silks, when combined with crop losses approached US$1000 million annually in the 1980s (Metcalf, 1986; Krysan and Miller, 1986). Since the introduction of the beetle into Europe at the beginning of the 1990s, economic losses have been recorded from Serbia, Hungary, Croatia, Romania, Italy and Austria. The damage potential in Europe is estimated at 472 million Euros annually, when no control measures are implemented (Wesseler and Fall, 2010).