Location: Beneficial Insects Introduction Research Unit
2022 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
Introductions of parasitoids from the regions of pest origin and breeding plants resistant to invasive pests have been effective at reducing invasive pest abundance and impact while providing safe, sustainable alternatives to the widespread use of insecticides. However, parasitoids and resistance are likely to interact in their impact on pests. The abiotic environment and in particular climate change is likely to affect these interactions. Research has shown that interactions between plant resistance and aphid virulence vary with temperature, as do interactions between aphids and parasitoids. Virulent biotypes can overcome plant resistance and plant resistance can affect the impact of natural enemies on herbivores. We developed mathematical models to address these interactions and parameterized them with published experimental results. We used these models to test these hypotheses: (1) parasitoids can reduce abundance of virulent aphids, (2) temperature regime affects the impacts of parasitoids on virulence frequencies, and (3) the outcomes these interactions vary with aphid species and crop. We modeled two systems: the soybean aphid, Aphis glycines, on soybean, and the Russian wheat aphid, Diuraphis noxia, on wheat. For the soybean aphid, we modeled the effects of parasitism by Aphelinus certus that arrived on its own in the U.S. soon after the soybean aphid invaded. For the Russian wheat aphid, we modeled the effects parasitism by Aphelinus atriplicis, which was introduced and established in the U.S. in the 1990’s to control this aphid. To make predictions about the impacts of climate change, we modeled dynamics under temperature regimes at the northern versus southern extremes of the U.S. geographical distributions of the aphids in the current climate. The idea is to use differences in geography as a surrogate for changes in climate that may emerge over time, the expectation being that temperature regimes will move northward as the climate warms. We found that the presence of parasitoids did reduce the abundance of virulent aphids, but temperature regime did not much affect the abundances of aphids in general or virulent aphids in particular. These results were similar for both soybean and wheat systems. This work addresses Project Plan Sub-Objective 2.B – Develop and test mathematical models of interactions between plants, aphids, and parasitoids in their effects on virulence frequencies.
Wheat and barley are major crops in the U.S. with annual revenues during the last decade of 9-19 billion dollars from 49-63 million acres planted. These values are 5-8 percent of total revenues and 16-20 percent of the planted acreage for all U.S. crops. The Russian wheat aphid came from Eurasia and was first detected on wheat in Texas in 1986. It then spread to 17 western states and became a pest of wheat and barley, infesting up to 2 million acres and costing up to $274 million per year. Resistant wheat and barley varieties were bred and deployed by the mid 1990s and provided effective control for a decade, however several virulent biotypes have been found in the US that can overcome this resistance. The Russian wheat aphid is seldom a pest in Eurasia, where parasites and predators can limit its abundance. Parasites and predators in the US had little impact on the Russian wheat aphid during its invasion, although generalist predators were found to reduce its abundance when aphid densities were high. Parasitic wasps (parasitoids) in the genus Aphelinus are among the most important Eurasian parasitoids of the Russian wheat aphid. Aphelinus species are important in biological control of invasive aphid pests. Aphelinus atriplicis, collected in the Caucasus region of southern Russia, was introduced into the U.S. in 1991 and has since become a major natural enemy of the Russian wheat aphid. Nonetheless, the aphid remains a sporadic pest, and Aphelinus hordei, a parasite of the Russian wheat aphid in Europe with a narrow host range, is a promising candidate for introduction. Aphelinus hordei can parasitize the aphid on wild grasses as well as wheat and barley, which can lead to fewer aphids surviving to colonize crops the following year. Furthermore, the parasitoids could slow the spread of virulent biotypes of the Russian wheat aphid. The North American Plant Protection approved a petition for its introduction, and APHIS-PPQ has approved a permit for field release in Colorado to control this pest aphid. In spring 2022, we reared and shipped aphids and parasitoids to our collaborator at Colorado State University (CSU). First we shipped ~50,000 avirulent D. noxia to our collaborator, who used them to infest an experimental field of susceptible wheat at the CSU Agricultural Research and Development Center. Two weeks later, we shipped ~100,000 A. hordei to our collaborator, who released them in the previously infested experimental field. Two weeks after release, our collaborator collected 48 wheat samples with mummified aphids and shipped them to our lab at Newark, Delaware. We have since collected adult A. hordei emerging from most of these samples, showing that the parasitoid was able to find, parasitized, and complete development on D. noxia in the field in Colorado. We will check over-wintering establishment by sampling in wheat fields in spring 2023. This work addresses Project Plan Sub-objective 3.C – Introduce parasitoids against pest aphids.
To better understand the genetic underlying differences in host specificity, we sequence, assembled, and annotate the genomes of four populations/species of Ganaspis near brasiliensis from Japan and China. This gave the following metrics for the assemblies of these populations/species based on PacBio Hi-Fi data assembled with the flye assembler (population, assembly size, N50, L50, and percent complete Busco Hymenoptera): G1_Yunnan, 1007 Mb, 872 kb, 333, 89%; G1_Tokyo, 1,010 Mb, 728 kb, 395, 89%; G3_Yunnan, 1,221 Mb, 251 kb, 1,202, 85%; G3_Nagano, 1,054 Mb, 661 kb, 472, 89%. The genome sizes are in line with the one for G. xanthopoda based on flow-cytometry. We captured the complete mitochondrial genomes on single contigs from each of the four assemblies. The lengths of these contigs are appropriate for the size on the mitochondrial genome and the contig sequences contain all the protein-coding genes and ribosomal-RNA coding sequences, and all but one or two transfer-RNA coding sequences (population, contig size, number of protein-coding genes, number of coding sequences for ribosomal RNA, number of coding sequences for transfer RNA, number of sites of replication origin): G1_Tokyo, 17,740 nt, 13, 4, 21, 5; G1_Yunnan, 17,162 nt, 13, 4, 21, 5; G3_Nagano, 18,030 nt, 13, 5, 22, 5; G3_Yunnan, 17,333 nt, 14, 5, 22, 5). Furthermore, feature orders match those for other figitid species. Based on sequence homology to the nr database at The National Center for Biotechnology Information (NCBI), the genome assemblies had less than 0.5 percent of bacterial sequence (0.2% in G1_ Tokyo, 0.2% in G1_Yunnan, 0.4% in G3 _Nagano, and 0.3% in G3_Yunnan). Most of this sequence was of Wolbachia species (100% in G1_Tokyo, 100% in G1_Yunnan, 90% in G3_Nagano, and 40% in G3_Yunnan), which was found on 4-5 contigs, the longest of which in each assembly contained what appears to the complete Wolbachia genome, based on lengths relative to the ranges for the reference assemblies of Wolbachia (1.3 Mb in G1_Tokyo, 1.4 Mb in G1_Yunnan, 1.3 Mb in G3_Nagano, and 1.2 Mb in G3_Yunnan). The presence of different strains of Wolbachia may explain reproductive incompatibilities between G1 and G3 populations. We analyzed for numbers of genes and of these the percent with blast hits, the percent with GO (gene ontology) mappings and annotations, and the percent with InterProScan domain annotations (G1_Tokyo had 61,193 genes and 81%, 65%, 33%, 95%, respectively; G1_Yunnan had 60 ,211 genes and 81%, 65%, 33%, 98%, respectively; G3_Nagano had 68,318 genes and 84%, 62%, 29%, and 82%, respectively; and G3_Yunnan had 75,068 and 83%, 63%, 30%, 98%, respectively). There were 2-3 times more genes in these assemblies than in those for Aphelinus species probably because of duplications which would also explain the rather large genome sizes. By mapping the reads from each population to the assemblies of the other populations reciprocally, we found the following densities of SNP (single-nucleotide polymorphism) loci per kilobase between for reads mapped to assemblies of of the other populations G1_Tokyo, G1_Yunnan, G3_Nagano, and G3_Yunnan: G1_Tokyo to G1_Yunnan = 5, G3_Nagano = 19, G3_Yunnan = 18; G1_Yunnan to G1_Tokyo = 4, G3_Nagano = 18, G3_Yunnan = 17; G3_Nagano to G1_Tokyo = 18, G1_Yunnan = 19, G3_Yunnan = 11; G3_Yunnan to G1_Tokyo = 19, G1_Yunnan = 19, G3_Nagano= 11) . The G1 versus G3 populations showed higher SNP density between them than within either G1 or G3 populations. Together with results on reproductive incompatibilities, this pattern indicates that the G1 and G3 populations are from different species. This work addresses Project Plan Objective 1: Determine the genetic basis of the host ranges and climatic tolerances of pest herbivorous insects and parasitoids of these pests.
To control the Russian wheat aphid, we introduced ~100,000 Aphelinus hordei into the field in Colorado and recovered adult A. hordei from the next generation. This parasitoid introduction should substantially improve control of the Russian what aphid and in particular virulent biotypes of the aphid that have overcome wheat.
Accomplishments
