A predator of potato on tomatoes

Charles A. Farrar 0 Thomas M. Perring 0 Nick C. Toscano

It may be a valuable natural control

The potato , a sometimes serious pest of Max Badgley tomatoes (above),can cause curling and stunting of leaves and stems, reducing fruit set and, in severe cases, killing the plant. The potato aphid is an occasional and ing, aphids excrete honeydew, which acts sometimes serious pest of tomatoes. Nat- as a growing medium for sooty mold. The ural enemies of the potato aphid, Macrosi- black-colored mold, on the foliage, re- phum euphorbiae (Thomas), include syr- duces the light available for photosynthe- phid larvae, ladybird beetles, lace- sis and, on the fruit, causes discoloration wings, and various hymenopterous and acts as a solar heat sink, increasing parasites, but these generally do the severity of fruit sunburn. not prevent economic damage. One natu- Despite the potential severity of dam- ral enemy seldom mentioned is the preda- age, economic threshold levels have not ceous midge, Aphidoletes aphidimyza been established for the potato aphid. (Rondani). In 1984-85 studies at the Uni- Control measures are begun when phys- versity of California South Coast Field ical damage to the plants becomes appar- Station, Irvine, we found that predation ent, and various chemical sprays effec- by larvae of this midge played an impor- tively reduce aphid numbers when tant role in controlling potato aphid infes- coverage is adequate. tations in processing tomatoes. Although the potato aphid only occa- sionally attains pest status, it is important The potato aphid to understand how its populations are usu- The potato aphid is relatively large, up ally kept under control in order to predict to 3 mm (%o inch) in length, and is either periodic damage. Many factors, including pink or green. Since the aphid has a wide fortuitous biological control, play a role in host range, alternative host plants are preventing most insects from becoming probably present as a source of infesta- pests. While biological control in row tion near most tomato plantings. Infesta- crops often has been thwarted by low tions occur when winged females migrate damage thresholds and the inherent insta- into a field and produce colonies of wing- bility of the cropping systems, the preda- less aphids. Each adult female produces ceous midge could be important in the about 30 aphids. natural regulation of potato aphid popula- The timing of these infestations is not tions. Preservation of this important related to tomato plant age. Unlike many predator would benefit tomato production other aphid species, the potato aphid is by (1) decreasing potato aphid damage most abundant during the summer, when and subsequent control costs, and (2) in a generation can be completed in as little reducing the amount of pesticide used for as eight days. Aphid colonies develop pri- potato aphid control, helping to preserve marily on the undersides of leaves, and natural enemies of other tomato pests. individuals feed by inserting their stylets into phloem tissue and withdrawing plant The cecidornyiid predator sap. Damage from the potato aphid takes Aphidoletes aphidimyza is very com- several forms and affects all tomato var- mon throughout the United States; the lar- ieties, but not to the same extent. Curling vae prey on at least 60 different aphid and stunting of leaves and stems, the most species. The delicate long-legged adult Larva of the predaceous midge A. aphido- obvious damage, is typical of that caused midge is brown, about 3 mm in length, myza may play an important role in controlling by many aphids. This damage reduces and feeds on honeydew excreted by potato aphid infestations. Aphids shown here fruit set and, if severe enough, can kill the aphids and other homopterous insects. It are dead from predation. plant. In addition, as a by-product of feed- is active in the afternoon and early eve-

CALIFORNIA AGRICULTURE, NOVEMBER-DECEMBER 1986 9 100 -1 0 100 10

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Jun Jul Aug Jul Aug SeP Fig. 1. Predatory midge population increased quickly to high levels in Fig. 2. In 1985, the were much less numerous and weren't found 1984, after which potato aphids decreased. until after potato aphids had reached high levels. ning, preferring a dark humid environ- only partially extracting the body fluids ment like that found near the soil surface of each aphid. Generally, larval develop- Conclusions and on the undersides of leaves. ment is accelerated when aphid density is Certainly a two-year study, one with The midge lays orange, elongate eggs, high. and the other apparently without the pre- about 0.3 mm long, singly or in groups of daceous midge, Aphidoletes aphidimyza, as many as 40 amid aphid colonies. The Experiments present during early potato aphid buildup, predaceous larva emerges in two to three Ideally, the effect of the predaceous does not constitute a valid check for the days and can crawl about 60 mm (about 2 midge should be tested by excluding it predator's effectiveness. Also, our obser- inches) in search of suitable aphid prey. It from potato aphid populations while leav- vation of predator abundance and activity feeds by piercing the soft tissue in the ing other elements of the natural enemy is not proof that the midge was the prima- aphid leg joints, injecting a toxin that complex unaffected. This is virtually im- ry cause of the 1984 aphid decline. How- paralyzes the prey, and sucking the body possible under field conditions, since mi- ever, our data and the midge's character- fluids. The larva takes 30 to 60 minutes to gration of any specific predator is diffi- istics, which are unique among the aphid consume one aphid, and in its lifetime can cult to control. Our evidence for effective predators, suggest that it is an important kill from 10 to 30 aphids, depending on the predation of the potato aphid by the predator of the potato aphid. species and age of the aphids. midge is based on field observations of the This predaceous midge has been stud- The orange larvae, which reach 4 mm two populations. ied throughout temperate regions, and it (XOinch) in length, are easily seen among In studies at the South Coast Field Sta- is now used in aphid management pro- live and shriveled dead aphids on a leaf. tion, we used seedlings of the processing grams in greenhouses, fruit trees, and row There is disagreement about the total tomato variety Peto-98 transplanted on crops. In outdoor crops, natural popula- number of larval stages, or instars, but June 18, 1984, and July 18, 1985, with 1.5- tions of the predator ,re monitored to after passing through at least three stages meter (5-fOOt) row and 45-cm (18-inch) predict their effectiveness in controlling lasting about four days each (at 70"F),the plant spacings. Plots 15 meters (50 feet) aphid infestations and, when feasible, larvae drop to the ground and pupate in long by four rows wide were replicated care is taken not to hinder predatory ac- the top 3 cm (1 inch) of soil. The adults five times. We made weekly counts of tivity. Evidence has been found of resis- emerge in 10 to 14 days, mate, and lay up aphids and natural enemies on one leaflet tance in this midge to organophosphate to 70 eggs in a one-week lifespan. from each of 25 plants per plot. The leaf- spray compounds, which would enhance Several elements of the midge's biol- lets sampled were fully expanded and its utility in pest management programs. ogy and behavior enhance its effective- were randomly selected from the upper The presence or extent of resistance in ness as a natural enemy of the potato third of the plant. any one population cannot be taken for aphid. The predator's wide geographic In 1984, the midge population in- granted, however. range and ability to feed on many aphid creased to high levels early, with a subse- In situations where this midge can be species ensure its presence in most toma- quent drop in the potato aphid population found early in a potato aphid infestion and to growing regions. A wide host range (fig. 1). The maximum average number of no other types of disruptive treatments would ordinarily decrease the probability aphids was 12 per leaflet with 0.8 midge are necessary, it is possible that the pred- of effective predation on one prey species, larva per leaflet. The potato aphid caused ator will be able to control the aphids un- but potato aphid populations grow in sum- no apparent damage to the tomatoes, and assisted. Further studies of the relation- mer when many other aphid species de- colonies of aphids which had been deci- ship between A. aphidimyza and the cline. This could enhance the importance mated by the midge were commonplace. potato aphid are warranted to provide in- of the potato aphid as a food source for Other natural enemies of the potato aphid formation needed to predict and enhance the midge and increase the likelihood of were seldom encountered. this natural control. early colonization of the aphid by the In 1985, we did not find the midge until predator. Adult predaceous midges also after the potato aphids had reached very Charles A. Farrar is Staff Research Associate, and can find aphids at low densities. They lay high levels, and even then its numbers Thomas M. Perring is Assistant Professor, Depart- eggs only in aphid colonies, and the num- were considerably below 1984 levels (fig. ment of Entomology, University of California,River- side. Nick C. Toscano is Program Director of Pest ber of eggs laid increases with increasing 2). The maximum numbers per leaflet Management and Entomologist, Cooperative Exten- aphid density. The larvae, too, exhibit were 80 potato aphids and less than 0.09 sion, UC Riverside. The authors thank G.P. Walker density-dependent behavior. When more predaceous midge. The tomatoes were and J.G. Morse for information regarding the biologies. This study was partially funded by the aphids are available than a larva can con- heavily damaged in spite of a late buildup Western Region Pesticide Impact Assessment Pro- sume, it will kill more individuals while of other natural enemies. gram.

10 CALIFORNIA AGRICULTURE, NOVEMBER-DECEMBER 1986