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The saliva that M. hirsutus injects into the host plant while feeding probably contains a substance that is phytotoxic (Williams, 1996). Host-plants differ in their susceptibility to the toxin. The more tolerant species tend to be infested at their growing points and in stem axils and infested new growth becomes stunted, with reduced internode extension and leaf expansion. Stunted stems may become swollen. In more sensitive plants, stunting is more marked and new growth forms cabbage-like clusters, with the mealybugs hidden in the creases of the growth. In highly susceptible plants, even brief probing of unexpanded leaves by crawlers causes severe crumpling of the leaves when they subsequently expand, while established infestation can cause total defoliation and even death of the whole plant. As the plant dies back from the tips, the mealybugs migrate to healthy tissue, so the colonies migrate from shoot tips to twigs to branches and finally down the trunk. Samanea saman is particularly severely affected.
It should be noted that the mealybug Paracoccus marginatus causes very similar damage on Hibiscus to that caused by M. hirsutus (Pollard, 1999).
Cryptolaemus montrouzieri, a native of Australia, has been used successfully to reduce large populations of M. hirsutus in Egypt, the Caribbean (Kairo et al., 2000), and India (Karnataka) (Mani and Krishnamoorthy, 2001). In Karnataka, India, on acid lime, two releases of 25 beetles per plant in January and February 1999 reduced the population of M. hirsutus to economically unimportant levels by mid-March (Mani and Krishnamoorthy, 1999); similarly, releases on guava reduced the mealybug population to insignificant levels within one month (Mani and Krishnamoorthy, 2001).
In Egypt, however, C. montrouzieri was unable to survive the cold of winter in sufficient numbers to be effective, and the main biological control agents there are the parasitoids Anagyrus kamali and Achrysopophagus sp. (Bartlett, 1978). In India, where grapes are grown in areas that may have quite a cold winter, the control agents used against M. hirsutus are the parasitoid Anagyrus dactylopii and the coccinellid predators Scymnus coccivora, S. conformis and S. gratiousus (Mani, 1989). Gowda and Manjunath (1998) reported that Hibiscus cannabinus was a suitable trap crop for M. hirsutus infesting mulberry in Mysore.
In Egypt, almost total control of the mealybug is maintained using the parasitoid Anagyrus kamali (Williams, 1996). This parasitoid has also been introduced to Grenada, Trinidad and some other Caribbean islands to control M. hirsutus (Pollard, 1995; Garland, 1998; Anon., 2000a; Michaud and Evans, 2000; Kairo et al., 2000). Pesticide spraying against disease vectors may reduce the natural enemy populations at times and allow a resurgence of the mealybug. Additional introductions of predators such as Cryptolaemus montrouzieri have been used on some Caribbean islands to reduce mealybug populations further (Gautam et al., 1996; Anon., 2000a). For biological control purposes, M. hirsutus can be reared in the laboratory on pumpkins, particularly those varieties with creases in the skin (Japanese pumpkin, Cucurbita moschata; acorn squash, Cucurbita pepo var. Turbinata) and on sprouting Irish potatoes (Mani, 1990; Meyerdirk, 1997; Serrano and Lapointe, 2002).
It has been found that damage to a territory newly invaded by M. hirsutus can be minimised if the pest can be identified quickly and biological control agents are introduced as soon as possible (Michaud and Evans, 2000; Kairo et al., 2000). The great success of the biological control programme against M. hirsutus in the Caribbean, using the predatory beetle Cryptolaemus montrouzieri and the hymenoptern endoparasitoids Anagyrus kamali and Gyranusoides indica, is largely attributable to these insects reproducing at least twice as fast as the mealybug (Persad and Khan, 2002; Meyerdirk and DeChi, 2005); populations were reduced by 82-97%, and the parasitoids were found to be effctive in tropical, subtropical and semi-desert conditions. Public awareness programmes were also important; public co-operation avoided heavy use of pesticides that might have impaired establishment of the biological control agents, and the public helped to disseminate the natural enemies (Kairo et al., 2000).
For Trinidad and Tobago, Singh (1999) estimated control costs of M. hirsutus in 1995-1997 to be US$ 5.1 million, while probable losses averted by control were estimated at US$ 41 million, giving a benefit:cost ratio of 8:1.
Colonies of M. hirsutus hidden in crevices amongst cabbage-like growths can be difficult or impossible for natural enemies to attack, especially for the larger coccinellid predators. This may limit the success of biological control agents in regulating pest populations because they cannot reach the mealybugs.
In Papua New Guinea, attendance by ants has been recorded to affect the level of attack of the mealybugs by parasitoids (Buckley and Gullan, 1991); the more aggressive the ant, the lower the level of parasitism observed. The effectiveness of natural enemies in regulating populations of M. hirsutus can be increased if ants attending the mealybugs can be controlled (Greve and Ismay, 1983).
In Egypt, the grape varieties Romi and Banati were found to be susceptible to attack by M. hirsutus, with the variety Moscati being the most tolerant and least affected (Amin and Emam, 1996).
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:
Williams (1996) summarizes records of damage caused by M. hirsutus. Almost all serious damage by the mealybug has been recorded between 7° and 30° North, where there are reports of seasonal differences in the incidence of the pest. Direct feeding on young growth (stems, leaves and flowers) causes severe stunting and distortion including crinkling of the leaves, thickening of stems and a bunchy-top appearance of shoots; in severe cases the leaves may fall. Honeydew and sooty mould contamination of fruit may reduce their value (Garland, 1998). In India, stunted and distorted growth caused by M. hirsutus in mulberry is known as Tukra disease (Rao et al., 1993) and is a problem in most of the silk producing areas (Tewari et al., 1994). It has been suggested that symptoms associated with M. hirsutus infestation may be due to a virus infection (on cacao in Zanzibar (de Lotto, 1967) and on mulberry in India (Tewari et al., 1994)).
Francois (1996) gave the estimated annual losses in Grenada due to M. hirsutus damage to crops and environment as US$ 3.5 million before biological controls were established. In the first few years of the mealybug problem in the Caribean, affected countries suffered serious loss of trade because other countries would not accept shipments of agricultural produce from them (Peters and Watson, 1999). In the period 1995-1998, Peters (1999) estimated the island's overall losses and costs at US$ 18.3 million, of which the control programme cost US$ 1.1 million (Kairo et al., 2000). Overall losses and costs to St Kitts in 1995-1997 were estimated by Francis (1999) as US$ 280,000, with an additional loss of trade estimated at US$ 22,000. For St Lucia, losses were estimated at US$ 67,000 (Anon., 1999), and for St Vincent and the Grenadines losses were estimated at US$ 3.4 million (Edwards, 1999). If the mealybug were to spread across the southern USA it is estimated that it could cause losses of US$ 750 million per year (Moffit, 1999).
Other crops seriously damaged by M. hirsutus include cotton in Egypt (Hall, 1921), with growth sometimes virtually halted; tree and herbaceum cotton in India (Dhawan et al., 1980; Muralidharan and Badaya, 2000), with reduction in yield; the fibre crops Hibiscus sabdariffa var. altissima (roselle), H. cannabinus (mesta) and Boehemeria nivea in West Bengal, India, and Bangladesh (Ghose, 1961, 1972b; Singh and Ghosh, 1970), with reduction in fibre yield of roselle of 21.4% reported by Ghose (1971a) and of 40% reported by Raju et al. (1988); grapes in India, with up to 90% of bunches destroyed in the Bangalore area (Manjunath, 1985) and heavily infested bunches made unfit for consumption or marketing (Vereesh, 1986); pigeonpea in India (Patel et al., 1990); Zizyphus mauritiana in India (Balikai and Bagali, 2000); ornamental Hibiscus in Papua New Guinea (Williams and Watson, 1988); and cacao in the Solomon Islands (Williams and Watson, 1988) and Grenada (Pollard, 1995).
In the Caribbean, damage has been reported on Annona spp., Spondias spp., okra (Abelmoschus esculentus), mango, sorrel (Hibiscus sabdariffa), Albizia saman and other ornamentals important to the tourist industry, and forest trees such as blue mahoe (Hibiscus elatus) and teak (Tectona grandis) (Pollard, 1995). Transport of fruit and vegetables between the Caribbean islands by entrepreneurs came to a virtual standstill with the imposition of quarantine restrictions on the importation of fresh produce into Trinidad (Pollard, 1995), although pre-export inspections have allowed imports from some affected countries to continue.
Experimental evidence suggests that Tukra-diseased leaves may be more nutritious to silkworms than normal leaves (Ahamed et al., 1999).