Location: Beneficial Insects Introduction Research Unit
2024 Annual Report
Objectives
Objective 1: Determine the genetic basis of the host ranges and climatic tolerances of pest herbivorous insects and parasitoids of these pests with a focus on using molecular genetic methods to elucidate factors responsible for the evolution of host specificity, to predict responses to climate change, and to develop methods for management of pest impacts. [NP304, C1, PS1A; C3, PS3A, 3B and 3C]
Sub-objective 1.A – Determine genetic basis of differences in host specificity of aphid parasitoids.
Sub-objective 1.B – Measure genetic variation in responses of parasitoids and their aphid hosts to different temperature regimes.
Sub-objective 1.C – Develop and test mathematical models for impact of climate change on population dynamics and evolution of parasitoids and aphid hosts.
Objective 2: Determine interactions between biological control, plant resistance, and aphid virulence in their effects on virulence frequencies. [NP304, C1, PS1A; C3, PS3A and 3B]
Sub-objective 2.A – Measure interactions between resistance, virulence, and parasitoids in their effects on virulence frequencies.
Sub-objective 2.B – Develop and test mathematical models of interactions between plants, aphids, and parasitoids in their effects on virulence frequencies.
Objective 3: Determine molecular phylogenetic relationships, test host specificity, and introduce parasitoids for biological control of target aphids. [NP304, C1, PS1A; C3, PS3A and 3B]
Sub-objective 3.A – Determine molecular phylogenetic relationships of parasitoids.
Sub-objective 3.B – Test host specificity of parasitoids of pest aphids.
Sub-objective 3.C – Introduce parasitoids against pest aphids.
Sub-objective 3.D – Measure the impact of the introduced parasitoids on distribution and abundance target and non-target aphid species.
Approach
Hypotheses about genetic architecture of host specificity have two extremes: (a) few genes with large effects that interact additively; (b) many genes of small effects that interact epistatically. We will take two approaches to testing these hypotheses. In one approach, we will use hybridization of A. atriplicis and A. certus in the laboratory to map QTL affecting parasitism of D. noxia, a host for A. atriplicis and a non-host for A. certus. We will cross A. atriplicis into A. certus with multi-parent advanced generation intercrosses and inbreed the intercrosses to make recombinant inbred lines (RILs). We will genotype SNPs in the RILs, make a linkage map, measure parasitism of D. noxia by these lines, and map QTL affecting parasitism. In the other approach, we will map QTL involved in differences between lines of A. rhamni reared for >140 generations on R. padi versus control lines reared on A. glycines. We will cross and backcross these lines, make a linkage map based on SNPs, measure parasitism of R. padi by backcross females, and map QTL affecting parasitism. We will test whether genes that diverge in sequence and/or expression between RIL and backcross females are associated with QTL found above. We will do this by genotyping RIL and backcross females for alleles in divergent genes and integrating these genes onto the QTL maps. To find genes that diverge in sequence or expression between A. atriplicis and A. certus and between selection and control A. rhamni, we have sequenced and assembled their genomes and transcriptomes. We will test whether divergent genes associated with QTL are expressed in sensilla on antennae, ovipositor, or mouth partsby hybridizing probes for the genes in parasitoids and imaging whole-mounts microscopically. We will whether divergence in gene sequences or expression levels correlate with differences in host specificity, host acceptance, and host suitability among Aphelinus species. We will measure genetic variation in temperature responses of parasitoids and their hosts that could allow adaptation to new temperature regimes using isofemale lines. We will develop and test mathematical models for the impact of climate change on population dynamics and evolution of parasitoids and hosts. To test this whether parasitoids slow the increase in frequency of virulent aphid genotypes that can overcome host plant resistance, we will do within-generation to get parameter estimates and multi-generation experiments to test the effects of parasitoids on viruluence freqencies. We will also develop and test mathematical models of interactions between plants, aphids, and parasitoids in their effects on virulence frequencies. We will continue to determine molecular phylogenetic relationships of parasitoids in the genus Aphelinus. We will continue to test the host specificity of parasitoids of pest aphids, and we will introduce parasitoids with narrow host range and measure their impact on the distribution and abundance target and non-target aphid species.
Progress Report
This is the final report for the Project 8010-22000-032-000D which ended February 2024. This project was consolidated with New project 8010-22000-031-000D, entitled “Biology, Ecology, Genetics, and Genomics of Introduced Species for Biological Control of Invasive and Other Insect Pests” to strengthen has been establishedcomplementary research efforts in biological control of invasive and other insect pests.
The accomplishments during the five years of Project 8010-22000-032-000D, documented by 10 published papers and four manuscripts in review, were as follows: Aphelinus rhamni, a candidate for introduction against the soybean aphid, Aphis glycines, responded rapidly to selection for parasitism of R. padi, a sub-optimal, low-preference host species. We tested the associations between single nucleotide polymorphisms (SNP) and parasitism of R. padi and found 28 SNP loci, some of which were associated with increased and others with decreased parasitism. We assembled the A. rhamni genome, mapped these SNP loci to the genome, and found association with 80 genes in 1.2 Mb of the 483 Mb genome of A. rhamni. We combined into a meta-assembly several assemblies of Aphelinus atriplicis, a major parasitoid of the Russian wheat aphid, Diuraphis noxia. Scaffolding the meta-assembly with genetic markers provided chromosomal-level assembly. Scaffolding an assembly of Aphelinus certus, an adventive parasitoid of the soybean aphid, with this A. atriplicis assembly, produced a the chromosomal-level for A. certus. We assembled the genomes and analyzed sequence divergence among them for five populations of Ganaspis near brasiliensis a parasitoid of spotted-wing drosophila, Drosophila suzukii, identified as the G1 versus G3 lineages based on cytochrome oxidase l sequences. We also made crosses to test reproductive compatibility between the G3 lineage from China and the G1 lineage from Japan. The combined results show that these lineages are different species so the decision to only introduce the more host-specific lineage was appropriate. Leptopilina heterotoma are obligate parasitoids of Drosophila species, and during oviposition, females inject venom that contains discrete, 300 nm-wide, mixed-strategy extracellular vesicles (MSEVs). We found 246 MSEV proteins in the L. heterotoma proteome, and assembly of the L. heterotoma genome revealed 90% of MSEV proteins are coded in the parasitoid genome. Simulations were done using mathematical models of two systems: the soybean aphid on soybean, the Russian wheat aphid on wheat, and associated parasitoids in the genus Aphelinus. The goal was to analyze the population dynamics of aphids and parasitoids on resistant versus susceptible plants, parameterized with published results. In the simulations, parasitoids reduced the abundances of virulent aphids, and on wheat, the percent of virulent aphids. The impacts of parasitoids on virulent aphids were greater in southern locations than in northern locations, and the impacts on virulent aphids were greater on wheat than on soybean. These results suggest that climate change would lead to decreased abundance and frequency of virulent aphids in northern locations. Aphelinus certus was freeze-intolerant in the laboratory with a median supercooling point of - 28°C. When exposed to temperatures of 0°C for up to seven months, adults emerged only after exposures of at least 60 days and survival decreased with durations beyond 150 days. At sites from northern Minnesota to southern Iowa, survival was negatively correlated with increasing latitude and was greater for parasitoids placed on the ground than 1 meter off the ground, likely due to the warmer and stabler temperatures of the subnivean microclimate. Conflicted signal in transcriptomic markers led to a poorly resolved backbone phylogeny of chalcidoid wasps (Hymenoptera: Chalcidoidea). The basal nodes of the phylogeny were strongly influenced by biased support from different functional gene complexes. We reviewed the use of reduced-representation libraries in insect genetics, and genetics, genomics, and speciation in Chalcidoidea.
Invasions by insect species that become pests are an increasing problem for agriculture. Introductions of parasitic wasps (parasitoids) from the regions of pest origin and breeding plants resistant to invasive pests can reduce their abundance and impact. Whether interactions between parasitoids and plant resistance are positive or negative depends on the detailed mechanisms. We studied the effects of susceptible versus resistant soybean and avirulent versus virulent soybean aphid on two species of parasitoids, one a generalist with a broad host range and one a specialist with a very narrow host range. The numbers of avirulent aphids parasitized by the generalist were lower on resistant versus susceptible soybean. However, the number of aphids parasitized by the specialist did not vary with plant resistance, aphid virulence, or their interaction. Emergence rates of wasp progeny were high for both the generalist and specialist and did not vary with plant resistance or aphid virulence. Progeny sex ratios of both wasp species did not vary with plant resistance or aphid virulence. However, for the generalist there was a small effect of the interaction between plant resistance and aphid virulence, with more males emerging from virulent versus avirulent aphids on resistant plants and fewer males emerged from virulent versus avirulent aphids on susceptible plants. Body sizes of female and male progeny of the generalist did not vary with plant resistance, aphid virulence, or their interaction. However, body sizes of female progeny of the specialist were larger on susceptible versus resistant soybean and on virulent versus avirulent aphids. Body sizes of male progeny of the specialist were larger on susceptible versus resistant plants. Given the higher densities of virulent versus avirulent aphids on resistant soybean reported in the literature, these parasitoids should parasitize more virulent than avirulent aphids on resistant soybean and would limit the abundance of virulent soybean aphid if much of the soybean acreage had resistant plants. Furthermore, A. glycines produced larger females on virulent aphids on resistant soy, and given that larger females are likely to have higher fecundity, the impact of A. glycines on virulent aphids on resistant wheat should increase over time.
Projects on biological control of invasive pests by introduction of natural enemies are sometimes faced with funding constraints that limit the numbers of individuals that can be introduced and the numbers of sites where they will be introduced. Using simulations with mathematical models, an analysis was done on the effects of the total numbers of insects introduced and their allocation among sites on rates of establishment, population growth, and spread of introduced species in stochastically varying environments. The approach was to use models of population dynamics where the rate of population growth in the environment of introduction varied randomly. The results describe the effects of the total number introduced and the number of introduction sites on the proportion of introductions that established, the net reproductive rate, and area occupied per generation, as affected by the spatial scale of the system, dispersal rate, and density-dependence of mating success. The results showed that introductions are unlikely to establish when the total numbers are introduced are low or when introductions are made at many sites, at least within the ranges of parameter values used in these simulations. Furthermore, net reproductive rate is likely to be low for low numbers introduced or when introductions are made at many sites. Lastly, introduced populations that do establish are likely to spread further when introductions are made at more sites. However, these conclusions only hold for the model structure and ranges of parameter values used in these simulations. Better estimates are needed for all parameters.
Accomplishments
1. Plant resistance, aphids, and parasitism.. Invasions by insect species that become pests are an increasing problem for agriculture, and introductions of parasitoids from the regions of pest origin and breeding plants resistant to invasive pests can reduce invasive pest abundance and impact. ARS research at Newark, Delaware, showed that virulent aphids able to overcome resistance were parasitized equally on resistant versus susceptible soybean, and in some cases more than avirulent aphids unable to overcome soybean resistance. Because parasitoids produced larger females on virulent aphids on resistant soybean and larger females are likely to have higher fecundity, parasitoid impact on virulent aphids should increase over time. Given the higher densities of virulent versus avirulent aphids on resistant soybean reported in the literature, these parasitoids should parasitize more virulent than avirulent aphids on resistant soybean, limit the abundance of virulent soybean aphid, and reduce its impact on soybean yield.