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Russian Wheat Aphid: Natural Enemies, Resistant Wheat Offer Potential Control Rz D

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Russian Wheat Aphid: Natural Enemies, Resistant Wheat Offer Potential Control Rz D Russian wheat aphid: natural enemies, resistant wheat offer potential control rz D. Gonzalez a Charles G. Summers o Calvin 0. Qualset u> 2 f Adult Russian wheat aphid. wears. Coordinated research to de- and social costs or the concerns associated velop economically and environ- with COmba~gRwA tati ions with insecticides. At present, insecticides are management the only RWA control tactic available to strategies for this pest is in California farmers. Continued use of these progress. agents will destroy native natural enemies, accelerate dev~opm~tof insecticide re- In March 1986, Russian wheat aphid sistance in RWA and other insect pests, (RWA),Diuruphis noxiu (Mordvilko),was and upset se~ndarypests. The results, first reported present in the United States, short-term, are induced pest outbreaks. causing crop losses in small grains in The results, long-term, are cont~ation Texas. Since then, it has spread, infesting of soil and watert wildlife losses, and gen- View of aphid from above showing characteris- small grains in Oklahoma, Kansas, Ne- eral en~o~ent~degradation. Economi- tic short antennae, double tail, and very short braska and South Dakota and moving cdy, too, the extensive acreage planted cornicles (small tubelike projections on each west to California, Oregon and Washing- with small gains, depressed markets and side of the aphid). ton. Crop losses in 1988 were an estimated decreased prices make unilateral chemical $130 million, 2.4 times greater than in protection a poor option. 1987. Losses since 1986 have exceeded ~u~he~ore,~~ea~edsmall grains $500 million. nonnally host native natural enemiesf pro- Russian wheat aphid is a small, pale viding natural control of potential p~ma~ green insect, often covered with a white and second^ pests in their crops and in powdery coating of wax. It prefers to colo- adjacent crops. ~~~ncingb~olo~cd con- nize areas deep in the whorl or beneath trol of RWA wiB ~e insecticide USE the leaf sheath, but as aphid numbers in- and permit maximum impact from native crease, the entire plant may be colonized. natural enemies in cereais and associated A cool-season aphid, it causes damage agroecosystems. from late fall through spring. While feed- After a 3-year exploration for RWA ing, RWA injects into the plant a toxic sa- natural enemies in Central Asia, D. liva that destroys chlorophyll, resulting in GonzAlez and others documented that white or cream-colored streaks on stems RWA has not been a pest on small grains. and leaves. The toxin also causes leaves to In these areas the aphid has been associ- twist and curl like a soda straw. ated with cereals for more than 50 years. In California, wheat and barley are ap- In the 3 years of exploration several poten- parently the most susceptible crops, fol- tially effective species (or biotypes) of lowed by rye and triticale. Oats also host natural enemies and many different poten- RWA, but suffer only marginal injury. The tially resistant wheat varieties have been RWA does not attack or injure corn, rice, identified from in and around Central or sorghum. It does, however, colonize Asia. These should be evaluated in the many native and introduced grasses. United States to advance biological controt The aphid first appeared in Imperial and host plant resistance of RWA. County in March 1988 and in Yolo County Our ongoing and proposed assessment the following May. It has since spread to of biological controI/hos~plant resis- all of California's major cereal-growing re- tance/chemical assessment (economic gions (fig. 1).Losses in California's cereal thresholds] are summarized here. The re- crops during the 1988-1989 growing sea- port reflects a summary of RWA research 9 son, including control costs, are estimated coordinated by UC researchers D. f at $8 million, according to the Great Plains Godezand C. G. Summers in biologicd Typical Russian wheat aphid damage showing Agricultural Council. Losses in rangeland control, C. G.Summers and C. 0. Qualset curled leaf and white streaking. grasses have not been determined. in host plant mistance, and C.G. Summers 32 ~ALIFORNIAAGRICULTURE, VOLUME 46, NUMBER 1 in the season and are rarely land valley, southern coastal valley, cen- inside curled leaves. There- tral inland valley and northern inland val- fore, effective biological con- ley. These represent the climatic zones in trol will require importing which most wheat is grown in California. new and exotic natural en- They also represent varied environmental emies from Central Asia, and climatic conditions under which the where natural enemies control impact against RWA of the same exotic this aphid. species and/or biotypes of natural en- Foreign exploration. Sci- emies can be assessed. entists in State Agricultural Experiment Stations (SAES), Migration and flight activity USDA-AgnculturalResearch With the aid of a statewide network of Service (ARS), and the USDA- suction traps (fig. l), UC scientists are Animal and Plant Health In- studying migration and flight activity of spection Service (APHIS) co- RWA. These studies are part of a larger operated in 1988-1991 to migration study being conducted through- import and establish natural out the aphid’s range. A pattern of migra- enemies of RWA in the tion and movement of winged aphids is United States. Foreign explo- emerging that will help determine ration has resulted in importa- whether planting dates can be manipu- tion and culture of more than lated within the constraints of field access, 124 source populations of wheat maturity and crop rotation to avoid natural enemies, including or escape these dispersal flights and thus parasites and predators. These prevent infestation when the crop is im- foreign explorations also pro- mature and most susceptible. While too duced information on (1) the few data have been collected to describe a frequent occurrence (in several locations in predictable pattern of aphid movement, a Fig. 1. Current distribution of Russian wheat several countries) in or near Central Asia small flight apparently occurs in fall and a aphid in California and location of suction of several parasite species routinely associ- much larger flight during spring (fig. 2). traps. ated with low densities of RWA, (2) effec- Occasionally, small flights occur in mid- tive parasitization of RWA under a wide winter. Planting so that cereals do not in migration and economic thresholds. range of climatic conditions, and (3) para- emerge until after the fall flight but mature Our studies are based on observations and sitization of RWA inside curled leaves. to at least flowering before the spring reports on RWA in fields in and around This information, plus experience with the flight may enable growers to avoid much Central Asia as well as on previous experi- imported natural enemies, causes us to be- of the wheat injury caused by RWA. Plant- ences in alfalfa with spotted alfalfa aphid lieve that five of the exotic parasites merit ing trials that encompass dates from Octo- and blue alfalfa aphid. Both alfalfa pests further study: D. rapae, Aphelinus asyckis, ber through February are underway at the initially caused extensive damage in al- Aphelinus varipes, Aphidius colemani and Kearney Agricultural Center and West falfa throughout the western United Apkidius matricarae. Side Field Station to obtain empirical data States, but the combination of biological Introduction of exotic parasites. The on aphid numbers and veld response as control and host plant resistance reduced five species of RWA parasites noted have they relate to time of planting and infesta- both pests to little economic siwcance. been released, in collaboration with tion. USDA-APHIS, in selective fields in Biological control California’s Imperial, Riverside, Santa Bar- Economic thresholds In California, biological control efforts bara, Fresno, Sacramento, and Yolo coun- Short-term, chemical intervention of- against RWA include a preliminary assess- ties, with the assistance of local farm advi- fers the most effective and immediate con- ment of native natural enemies attacking sors. Sites for release were selected on the trol of RWA. Development of economic RWA before introduction of exotic en- basis of variations in different habitats and thresholds for RWA is underway at the emies; foreign exploration to determine climatic patterns in the following growth Kearney Agricultural Center as part of a the type and extent of mortality factors as- zones: southern low desert, southern in- cooperative Western Regional project. sociated with RWA outside the United 500 - These thresholds will be dynamic and re- States, where this species has existed in flect the plant growth stage (age) at initial 450 - --,-Westside - Fresno Co. low numbers for a long time, and prelimi- 13- Chico - Butte Co. infestation and the degree of aphid dam- 400 - + Davis - Yolo Co. nary selective introduction of five exotic 2 350- + Lancaster - Lo9 Angeles Co. age (number of tillers infested) and aphid parasite species. 300- density. Preliminary data show that aphid Surveys for native natural enemies. $ 250- infestations on young plants can result in Indigenous primary parasites attacking 2 200 - extensive plant mortality and that plants RWA in the United States include 150- are susceptible to injury and yield loss as Lysiphlebus testaceipes, Diaeretiella rapae and 100 late as head emergence (table 1). Apkelinus varipes. Several species of 50 Russian wheat aphid is susceptible to coccinellid and dipterous predators also 0 of the common insecticides used for Jan Feb Mar Apr May Jun JuI Aug Sep Oct all attack RWA and other aphids-on small Sample date aphid control in small grains; there are no grains. However, the extant aphid natural Fig. 2. Flight pattern of Russian wheat aphid known cases of aphid resistance. The enemy species in the United States do not representing the high desert (L.A. Co.), San aphid is, however, difficult to control be- effectively control RWA.
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