2013 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 sequenced and assembled the genomes of APHELINUS ATRIPLICIS and APHELINUS CERTUS, which are aphid parasitoids. The assemblies are 267 and 277 megabases long, respectively, and these sizes are within 80-87% of the sizes estimated from flow cytometry. The assemblies include 98-99% of 248 highly conserved genes in the core eukaryotic gene-mapping approach, and have 22-24k putative genes which match 19x genes in GenBank. We also sequenced the transcriptomes of these species, and the data mapped to 21-22k genes. We mapped sequences from A. CERTUS to the 21k A. ATRIPLICIS genes supported by mRNA data. Analysis of sequence divergence revealed 459 genes under selection; 30 percent had no known function but may underlie differences in host specificity. We plan experiments on tissue specific expression and gene knockout to determine gene function. This research relates to objective 1.1 of the project.
We found 11 strains of endosymbiotic bacteria in 6 genera in 44 populations of the aphid cowpea aphid, APHIS CRACCIVORA. However, prevalences were lower than in other well-studied aphids. ARSENOPHONUS was prevalent in cowpea aphid on locust but rare or absent on other plants; HAMILTONELLA was prevalent in cowpea aphid on alfalfa but rare or absent on other plants. HAMITONELLA provides protection against some parasitic wasps, but ARSENOPHONUS does not, so aphids on different host plants may have different levels of parasitism. This research relates to objectives 1.4 of the project.
With Texas A&M University, we continued revision of the systematics of APHELINUS, for which we received funding from a National Science Foundation grant. We have sequenced DNA libraries for 42 populations in 11 species of APHELINUS and are now analyzing these data for single nucleotide polymorphisms to generate phylogenetic trees. This research relates to objective 2 of the project.
We produced ~20,000 A. GLYCINIS and shipped them to Minnesota in fall 2012, where colleagues released them on soybean aphid on buckthorn. We will release larger numbers on soybean aphid in Minnesota soybean fields in summer 2013. We established a culture of another population in the APHELINUS VARIPES complex from collections by Institute of Plant Protection (Chinese Academy of Agricultural Sciences) during fall 2012 in Shandong Province 400 km to southeast of Beijing. We started host specificity testing of APHELINUS from collections in China, 2010 and 2012, and Korea, 2010. This research relates to objective 3.1 of the project.
We conducted additional host specificity experiments on APHELINUS HORDEI, a candidate for introduction against the Russian wheat aphid, DIURAPHIS NOXIA, a major pest of small grains in the western US, and are now analyzing the data. This research relates to objective 3.2 of the project.
In anticipation of receiving parasitoids from Asia for introduction against spotted-wing drosophila, DROSOPHILA SUZUKII, a major new pest of small fruits in the US, we started cultures of this fruitfly. This research relates to objective 3.3 of the project.
Bacteria living in aphids do not protect aphids from attack by parasitic wasps used in biological control. APHELINUS species are parasitic wasps used in biological control of aphid species. Bacteria living in aphids have been shown to reduce parasitism of aphids by other parasitic wasps used in biological control, but their effect on APHELINUS species is unknown. ARS researchers at Newark, Delaware, tested the effect of HAMITONELLA DEFENSA, a bacterium in the cowpea aphid, APHIS CRACCIVORA, on parasitism by APHELINUS GLYCINIS, APHELINUS ATRIPLICIS and APHELINUS CERTUS. Neither parasitism nor adult size of progeny were affected by the presence of H. DEFENSA. This bacterium does not confer resistance to aphelinid parasitoids of aphids, which means that these parasitic wasps will be able to control aphid pests regardless of the presence of defensive bacterial symbionts.
Asian parasitoid released against soybean aphid. Soybean aphid is a major pest of soybean in the US, in part because it arrived with out its coevolved parasitoids. APHELINUS GLYCINIS is a parasitoid from China that has been approved for introduction against soybean aphid after a decade of exploration and host specificity testing of more than 40 parasitoid populations in over 15 species. ARS researchers at Newark, Delaware, reared 40,000 APHELINUS GLYCINIS, and together with colleagues at the University of Minnesota, released them in soybean fields in St. Paul, Minnesota. These parasitoids have established within the season and are parasitizing large numbers of the soybean aphid.
Soybean resistance to soybean aphid reduces, but does not eliminate, attack by parasitic wasps used in biological control. Host plant resistance to soybean aphid may interfere with parasitism by parasitic wasps being introduced to control soybean aphid. ARS researchers at Newark, Delaware, tested whether soybean-aphid resistant soybean varieties affected parasitism of the soybean aphid by APHELINUS GLYCINIS, which is being introduced from China to control soybean aphid. Two resistance genes reduced the number of soybean aphids parasitized by APHELINUS GLYCINIS, but this parasitic wasp was still able to parasitize soybean aphid on soybean-aphid resistant soybean. These results show that host plant resistance and biological control by APHELINUS GLYCINIS are partially compatible.
Hopper, K.R., Prager, S., Heimpel, G.E. 2013. Is parasitoid acceptance of different host species dynamic?. Functional Ecology. 10.1111/1365-2435.12107.
Kuhn, K.L., Duan, J.J., Hopper, K.R. 2013. Next-generation genome sequencing and assembly provides tools for phylogenetics and identification of closely related species of Spathius, parasitoids of Agrilus planipennis (emerald ash borer). Biological Control. 66:77-82.