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United States Department of Agriculture

Agricultural Research Service

2010 Annual Report

1a.Objectives (from AD-416)
(1) Determine the genetic and evolutionary basis for host specificity of insect parasitoids and herbivores; (2) Determine the importance of climatic adaptation for establishment and growth of introduced populations of insects; and (3) Screen, introduce, and evaluate impact of candidates for biological control introductions, based on host specificity and climatic tolerances.

1b.Approach (from AD-416)
For the first objective, we will determine the genetic basis for a host shift in the specialist herbivore, Heliothis subflexa, and for differences in host specificity between species in the Aphelinus varipes complex using crosses, quantitative trait loci mapping, and differences in gene expression. We will confirm gene function by silencing with RNA interference. Climate matching is frequently used to decide where to collect biocontrol agents for introduction. However, three hypotheses can explain climatic adaptation:(1) populations in different regions are adapted to local climates, (2) single populations have the full range of genetic variation in traits affecting climatic adaption, and (3) physiological plasticity is sufficient for local adaptation. These hypotheses have very different implications for collection strategies. The second objective is to test these hypotheses using the A. varipes complex. In the third objective, we will use the knowledge and methods developed under objectives 1 and 2 to screen candidates proposed for introduction to control Diuraphis noxia and Aphis glycines, introduce the most promising candidates, evaluate their impact on target and non-target species, and determine whether the screening was useful in improving the success and safety of biocontrol introductions.

3.Progress Report
This is the final report for 1920-22000-017-00D which terminated (6/14/2010). The project was replaced by a bridging project 1920-22000-021-00D.

Progress was made on all project objectives, which address National Program 304. Host specificity is crucial for safe biocontrol introductions, but the likelihood of host specificity evolution depends on its genetic architecture. Using crosses between species in the APHELINUS VARIPES complex, we found only two QTL explained significant variation in parasitism of Russian wheat aphid. This is the first analysis of the genetic architecture of parasitoid host specificity. Using interspecific crosses of two closely related moth species, we showed that changes in alternative genes enabled moths to use novel host plants. These results suggest a greater likelihood of host shifts than previously thought.

Host specificity may vary with parasitoid age or physiological state. Contrary to this hypothesis, the host range of APHELINUS near GOSSYPII was stable when it was aged or starved. This suggests experiments on host range need not test effects of age or physiological state in species with narrow host ranges, which makes experiments cheaper and easier.

Determining the source of introduced biocontrol agents is needed to evaluate success and safety. In the 1990’s, aphelinid species were introduced against the Russian wheat aphid. Recent collections in western states showed this aphid is now heavily parasitized by APHELINUS ATRIPLICIS from the Caucasus, which we determined using DNA and morphological data. This not only shows that the introduction program was successful but also lets us test whether specificity has evolved since parasitoid introduction.

Cryptic species look similar but often differ in host specificity so confounding them can affect the success and safety of biocontrol. We have identified cryptic species in the APHELINUS VARIPES complex based on DNA, behaviorial, and morphological data. Parasitoids in the APHELINUS MALI complex are important in aphid biocontrol, but species identification is difficult because comprehensive keys are not available. We developed computer-based taxonomic keys for both complexes, which will increase safety and success of introductions.

Soybean aphid has become the major pest of US soybean, but we have found promising candidates for its biocontrol. We have imported and reared in quarantine 46 populations of soybean aphid parasitoids from Asia and evaluated host specificity of 28 populations; 24 had broad host ranges and have been dropped, but 3 species had narrow host ranges. Permission for introduction one of these, BINODOXYS COMMUNIS, has been obtained and it has been introduced. Two species new to science, APHELINUS near GOSSYPII and APHELINUS near ENGAEUS from China, have narrow host ranges and petitions are being submitted for their introduction. Establishment of these parasitoids should reduce abundance of soybean aphid and thus damage and control costs.

1. Promising candidates for biological control of soybean aphid discovered. Since soybean aphid was accidentally introduced into the U.S. around 2001, it has become the major pest of soybean. To find biological control agents to introduce against soybean aphid, ARS researchers at Newark, DE have imported and reared in quarantine 46 populations of soybean aphid parasitoids in at least 15 species from China, Japan, and Korea during the last 8 years; 4 new populations were imported in 2010. We have evaluated host specificity of 28 populations; 24 had broad host ranges and have been eliminated from consideration. Three species were previously found to have narrow host ranges. This year another species new to science, APHELINUS near ENGAEUS from Xiyuan, China, was found to have a narrow host range. These results support the safety of introducing this parasitoid to control soybean aphid. Establishment of this parasitoid should significantly reduce abundance of soybean aphid and thus damage and control costs.

2. Developed key for identification of parasitoids in APHELINUS MALI species complex. Parasitoids in the APHELINUS MALI species complex are important in biological control of aphid pests. For example, ARS researchers in Newark, DE have found two Asian species new to science in this complex that are promising candidates for introduction against soybean aphid. However, identification of species in the complex is difficult because comprehensive keys have been unavailable. We have developed a computer-based, multiple-entry taxonomic key for the complex, based on published species descriptions and examination of material collected during the project on biocontrol of soybean aphid. This key will greatly facilitate identification of the closely related species in this complex and thus improve the safety and efficacy of biological control using these parastitoids.

3. Stable parasitoid host range is not affected by stress. Host specificity is a critical trait in candidates for biological control introductions. However, it has been hypothesized that host specificity of parasitoids may vary with their experience or physiological state. Contrary to this hypothesis, the host range of APHELINUS near GOSSYPII, a candidate for introduction against soybean aphid, was stable when the parasitoids were old or starved. This suggests that one need not account for experience or physiological state when testing parasitoid host range, at least in species with narrow host ranges, which means that host range evaluation experiments will be cheaper and easier.

Review Publications
Heimpel, G.E., Frelich, L.E., Landis, D.A., Hopper, K.R., Hoelmer, K.A., Sezen, Z., Asplen, M.K., Wu, K. 2010. European buckthorn and Asian soybean aphid as components of an extensive invasional meltdown in North America. Biological Invasions. DOI: 10.1007/s10530-010-9736-5.

Hopper, K.R., Oppenheim, S.J. 2009. GENETICS OF HOST RANGE IN LEPIDOPTERA. In: Goldsmith, M., Marec, F., Editors. Molecular Biology and Genetics of Lepidoptera. Boca Raton, Florida: CRC Press. 195-217.

Last Modified: 4/22/2015
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