2012 Annual Report
1a.Objectives (from AD-416):
1 - Determine the behavioral, physiological, and genetic basis for host specificity of parasitoids of pest insects, such as soybean aphid (Aphis glycines) on soybean, Russian wheat aphid (Diuraphis noxia) on small grains, cherry vinegar fly (Drosophila suzukii) on soft fruits, and other invasive insects, and of insect herbivores, such as noctuid moths in the genus Heliothis on cotton and other crops.
1.1 – Determine the genetic architecture underlying differences in host
1.2 – Test whether parasitoids introduced against insect pests have evolved
to parasitize endemic species and whether parasitoids resident prior
to introductions have evolved to parasitize exotic pests.
1.3 – Test how the host acceptance behavior of parasitoids depends on their
physiological state, aphid defenses, and ecological factors.
1.4 - Determine the role of bacterial endosymbionts in protecting insects
2 - Determine the identity and phylogenetic relationships among species of parasitoids proposed for introduction against pest insects, such as soybean aphid, Russian wheat aphid, and cherry vinegar fly.
3 - Screen, introduce, and evaluate impact of candidates for biological control introductions against pest insects, such as the soybean aphid on soybean, the Russian wheat aphid on small grains, cherry vinegar fly on soft fruits, and other invasive insects.
3.1 – Biological control introductions against soybean aphid.
3.2 – Biological control introductions against Russian wheat aphid.
3.3 – Biological control introductions against cherry vinegar fly.
1b.Approach (from AD-416):
Safe and effective biological control depends on using narrowly host-specific agents, and host specificity depends on internal physiological state and previous experience, as well as ecological factors. Furthermore, the likelihood that a host-specific insect will evolve to attack a novel host species depends on the genetic architecture of host use. We will determine the behavioral and genetic bases for host shifts in the herbivores in the genus HELIOTHIS and aphid parasitoids in the genus APHELINUS using laboratory experiments on host use behavior and quantitative genetics/genomics analyses of inter and intraspecific crosses to map and identify genetic architecture. We will also test whether introduced aphid parasitoids have evolved to parasitize endemic species and whether aphid parasitoids resident prior to introductions have evolved to parasitize invasive pests. To do this, we compare host ranges parasitoids before and after introduction in laboratory experiments. Cryptic species are closely related species that differ little in the morphology, but differ critically in traits like host specificity, and studies on host range have often confounded cryptic species. Using modern methods of molecular phylogenetics, morphometrics, and behavioral biology, we develop robust phylogenies and identification keys for species complexes of aphid parasitoids important in biocontrol. We will use the knowledge and methods from the above research to screen specificity of candidates proposed for biocontrol introductions against soybean aphid, Russian wheat aphid, and other invasive pests. We will introduce the most promising candidates and evaluate their impact on target and non-target species with field exclosures, surveys, and analyses of population dynamics.
We continued mapping the genes affecting host specificity in APHELINUS and HELIOTHIS species. We have sequenced and assembled genomic libraries for APHELINUS ATRIPLICIS, APHELINUS CERTUS, HELIOTHIS VIRESCENS and HELOTHIS SUBFLEXA. We are using the assembled genomes as references for genotyping single nucleotide polymorphisms to use in genetic mapping of quantitative trait loci and in phylogenetic analyses of relationships. Using a new protocol for reduced representation libraries of genomic DNA that allows genotyping by sequencing, we have generated and are sequencing libraries of individuals from crosses to use in genetic mapping of loci involved in host specificity.
An experiment on the effect of an endosymbiotic bacteria of aphids (HAMITONELLA DEFENSA) on resistance to parasitism by APHELINUS GLYCINIS, a candidate for introduction against soybean aphid, showed that although parasitism was not affected, adult body size of parasitoid progeny was smaller in HAMITONELLA-infected than in HAMITONELLA-free APHIS CRACCIVORA of the same genotype. We carried out an experiment to confirm this result using A. GLYCINIS and APHELINUS CERTUS exposed to both APHIS CRACCIVORA and ACYTHROSIPHUM PISUM.
We continued with revision of the systematics of the genus APHELINUS in cooperation with Texas A&M University. Using reduced representation DNA libraries for 15 populations in 9 species of APHELINUS, we genotyped 271,095 single nucleotide polymorphisms, 49,697 of which were phylogenetically informative. Using these data, we generated a phylogenetic tree that was completely resolved with 100 percent bootstrap support. We have submitted a proposal to the National Science Foundation to support research on the revision of the genus APHELINUS using a combination of morphometrics and DNA sequence data.
For biological control of soybean aphid and Russian wheat aphid, we continued to rear parasitoid cultures and test their host specificity. We finished host specificity testing of APHELINUS COREAE, a specie from Korea new to science, which has a sufficiently narrow host range for introduction to control soybean aphid. We submitted a petition to the North American Plant Protection Organization for introduction of APHELINUS RHAMNI to control soybean aphid and the petition was approved. On the basis of this petition and additional data, we submitted an application to APHIS-PPQ for the release of A. RHAMNI. We also submitted application to APHIS-PPQ for the release of A. GLYCINIS. We plan to release these parasitoid species in Minnesota in 2013 to control soybean aphid. We determined that our cultures of these species are still capable of winter diapause, which means that they should be able to overwinter in the northern mid-western soybean growing regions where it will be introduced. We obtained renewed funding from the North Central Soybean Research Program to support the work on soybean aphid biocontrol.
New species of parasitoids described and key developed. During exploration for parasitoids of soybean aphid in Asia, ARS researchers at Newark, Delaware, discovered and described three species that are new to science. These are APHELINUS COREAE, APHELINUS GLYCINIS, and APHELINUS RHAMNI, and they are members of the APHELINUS MALI species complex. We also developed a key for to all the species in the APHELINUS MALI complex, which will greatly aid in identification of these difficult to distinguish species. These descriptions and key will allow identification of these new parasitoid species after introduction and thus aid it evaluating their impact on soybean aphid abundance.
Petition approved for introduction of soybean parasitoid. Soybean aphid has become a major pest in the US causing damage to as much as 70 million acres of soybean crop per year. Biological control, using Asian parasitoids specific to soybean aphid, is a promising approach to control of this invasive soybean pest. After testing the specificity of 21 populations of Asian parasitoids during 10 years, ARS researchers at Newark, Delaware, identified the parasitic wasp APHELINUS RHAMNI as effective and safe for introduction. The North American Plant Protection Organization has approved a petition for introduction of this parasitoid into the midwestern US. We have applied for USDA permit to introduce A. RHAMNI, and a permit to introduce another parasitoid species, APHELINUS GLYCINIS, has been approved. Introduction of these biological control agents could greatly reduce damage by soybean aphid to US soybean production.
Hopper, K.R., Woolley, J.B., Hoelmer, K.A., Wu, K., Lee, S. 2012. An identification key to species in the mali complex of Aphelinus (Hymenoptera, Chalcidoidea) with descriptions of three new species. Journal of Hymenoptera Research. 26:73-96.
Oppenheim, S.J., Gould, F.L., Hopper, K.R. 2012. The genetic architecture of a complex ecological trait: host plant use in the specialist moth, HELIOTHIS SUBFLEXA. Evolution. DOI: 10.1111/j.1558-5646.2012.01712.x.