2009 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.
We continued with development of genetic maps for analysis of host specificity in APHELINUS species and HELIOTHIS species. On biological control of soybean aphid and Russian wheat aphid, we continued to collect and establish cultures of parasitoids in quarantine and test their host specificity.
Source of parasitoids attacking Russian wheat aphid was identified. Determining the source of introduced biological control agents is necessary to evaluate the success and safety of biological control projects. A decade ago, parasitoids in the genus APHELINUS were released to control the Russian wheat aphid, a major pest of wheat and barley crops in the western states. Using DNA sequence data and morphological analyses, we determined that the APHELINUS species that now heavily parasitizes Russian wheat aphid in Colorado, Nebraska, and Wyoming is APHELINUS ATRIPLICIS from the Caucasus region of Russia and the Republic of Georgia. This knowledge not only shows that the introduction program was successful but also allows us to test whether host specificity has shifted since introduction of this parasitoid.
High parasitism by introduced parasitoid of the Russian wheat aphid may cause area-wide suppression. The Russian wheat aphid is a major pest of wheat and barley crops in the western states. Parasitoids in the APHELINUS VARIPES complex were released a decade ago to control the Russian wheat aphid. Collections in Colorado, Nebraska, and Wyoming showed that Russian wheat aphid is now heavily parasitized by species in the APHELINUS VARIPES complex. ). Establishment of these parasitoids should significantly reduce abundance of the Russian wheat aphid and thus damage and control costs.
Plasticity in genetic make-up that allows insects to switch host plants indicates potential for rapid evolution host specificity. A major question in biological control is whether a plant-feeding insect imported to control a weed is likely to switch to attack other plants and adversely affect agriculture or the environment. Using genetic markers mapped to 31 chromosomes in two closely related moth species, we found that genes on different chromosomes enabled moths to develop on a novel host plant, showing that the same behavior can be produced by changes in different genes. These results suggest that the likelihood of host plant shifts in plant-feeding insects are greater than previously thought and have implications not only for the safety of biological control introductions but also for shifts in the ability of pests to feed on previously unattacked plant species.
Anchored genetic markers for moth chromosomes provide useful tool for moth genomics. Markers for genetic mapping are important tools for analysis of genes involving host specificity, climatic adaptation, and other traits. Such markers are particularly useful if their location and homology can be determined. Using published sequences for known genes in HELIOTHIS VIRESCENS, we developed 52 molecular markers and integrated these into a genetic map based on amplified fragment length polymorphisms. The known genes map to 22 of the 31 chromosomes in HELIOTHIS VIRESCENS. These markers will be critical to our analysis of the genetic architecture of host specificity, and very useful to the scientific community working on these moths.
Promising candidates for biological control of soybean aphid discovered. Since soybean aphid was accidentally introduced into the US around 2001, it has become the major pest of soybean. To find biological control agents to introduce against soybean aphid, we have evaluated host specificity of 24 populations of parasitoids in 10 species from China, Japan, and Korea during the last 7 years. Twenty had broad host ranges and have been eliminated from consideration, but four had narrow host ranges. Permission has been given to introduction one of these, BINODOXYS COMMUNIS, which has been released in the field. A second species new to science, APHELINUS near GOSSYPII from Beijing, readily accepts and survives well in soybean aphid, but survives much less well in aphids the genus APHIS, and hardly recognizes aphids in other genera. These results support the safety of introducing this parasitoid to control soybean aphid. Establishment of these parasitoids should significantly reduce abundance of soybean aphid and thus damage and control costs.
Desneux, N., Barta, R.J., Hoelmer, K.A., Hopper, K.R., Heimpel, G.E. 2009. Multifaceted determinants of host specificity in an aphid parasitoid. Oecologia. 160:387-398.
Fauvergue, X., Hopper, K.R. 2009. French wasps in the New World: experimental biological control introductions reveal a demographic Allee effect. Population Ecology. 51(3):385–397