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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 and degree of response 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 infestations 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 on Hibiscus, the mealybug Paracoccus marginatus causes very similar damage 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 (Psidium guajava) 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 (Vitis vinifera) 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. gratiosus (Mani, 1989). Gowda and Manjunath (1998) reported that Hibiscus cannabinus was a suitable trap crop for M. hirsutus infesting mulberry (Morus) in Mysore.
In Egypt, almost total control of the mealybug is maintained using the parasitoid A. 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; Kairo et al., 2000; Michaud and Evans, 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 C. 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 (Cucurbita), particularly those varieties with creases in the skin (Japanese pumpkin, Cucurbita moschata; acorn squash, Cucurbita pepo var. turbinata) and on sprouting Irish potatoes (Solanum tuberosum) (Mani, 1990; Meyerdirk, 1997; Serrano and Lapointe, 2002).
It has been found that damage to a territory newly invaded by M. hirsutus can be minimized if the pest can be identified quickly and biological control agents are introduced as soon as possible (Kairo et al., 2000; Michaud and Evans, 2000). The great success of the biological control programme against M. hirsutus in the Caribbean, using the predatory beetle C. montrouzieri and the hymenopteran endoparasitoids A. 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 De Chi, 2005); populations were reduced by 82-97% and the parasitoids were found to be effective 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: