Conrad Berube/Island Crop Mgt. HTML info

Conrad Bérubé
island crop management
email: uc779(at)freenet.victoria.bc.ca


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Increasing concern over the safety, efficacy and sustainability of chemical control of insect pests has spurred research in alternate tactics in an strategy of integrated pest management. Much interest has been generated in regard to biological control, particularly for those introduced pests whose complete pool of natural enemies may not have been transplanted with them to new regions. Biological control is particularly appropriate for control of insect pests in urban settings since, aside from any ecological upset that pesticides might cause, applications of chemical agents would certainly meet with considerable opposition from city residents. In addition, once an outbreak has been reduced to tolerable levels, introduced biological control agents can maintain long-term control-- in other words tactics of this sort represent a one-time investment, in contrast to costs associated with repeated application of pesticides.

First discovered in California in 1988, the ash whitefly ( Siphoninus phillyreae), a native of Eurasia and northern Africa, is a homopteran pest of a number of commercial crops and ornamentals. Most notably, in the Central Valley and Bay Area, the whitefly can be found on shamel ash (Fraxinus uhdei), ornamental pear (Pyrus calleryana), Citrus species and toyon (Heteromeles arbutifolia). The immature whiteflies produce copious amounts of honeydew that falls on foliage, automobiles, sidewalks, and lawns and can be carried via foot traffic and air dispersion, into businesses and homes. The sticky, unsightly honeydew and accompanying sooty mold is, at best, a nuisance and its accumulation may cause defoliation and death of the infested host plant if the whitefly is left unchecked. Numbers of the adult pests in flight have attained such proportions as to have been likened to snow flurries, reducing the quality of life by limiting outdoor activities.

Infestations of the whitefly have been reduced in southern California by the introduction of a miniwasp, Encarsia partenopea, reared at UC/Riverside in a program coordinated by the California Department of Food and Agriculture. Females of the tiny wasps lay their eggs in immature whiteflies. The wasp egg hatches inside the whitefly "pupa", parasitizing and eventually killing it. Instead of a whitefly emerging from the pupa an Encarsia miniwasp will cut its way out. The City of Davis and the University of California/Davis have cooperated in a joint venture to rear these parasitoids for releases that supplement the introductions made the previous year.

Few pesticides are compatible with biological control but, other control measures may complement control of the ash whitefly:

Ants are important predators of many arthropod pests. However, ants are attracted to honeydew produced by whiteflies. When collecting honeydew, ants may disturb parasitoids and discourage their oviposition. Therefore it may be helpful to prevent ants from climbing into infested trees. One way this can be accomplished is to tie a narrow band of terricloth rag around the tree, smearing the outside of this band with an adhesive designed to trap insects. (One such product, Tanglefoot, is available in most garden-supply outlets. It is best not to apply the adhesive directly to the trunk as this may lead to problems of bark rot.) Make sure that tall grass or low-hanging branches do not afford alternate routes for ants into the canopy.
Pruning the canopy of heavily infested host plants can increase sunlight and air circulation around the immature whiteflies causing many of the pests to dry up and die.
The opposite tack of spraying water up into infested branches may drown some of the immature whiteflies.

It is best to complete the above stratagems before releasing the parasitoids. Release of reared parasitoids is simple: the tube in which the wasps are delivered is opened and then taped or wedged in a tree infested with whiteflies. The tube may be removed when all of the Encarsia have found their way out (usually within a few hours).


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