ESTIMATION OF HYMENOPTERAN PARASITISM IN CEREAL APHIDS BY USING MOLECULAR MARKERS

Authors: JONES, DOUGLAS - OSU, DEPT ENTOMOLOGY; GILES, KRISTOPHER - OSU, DEPT ENTOMOLOGY; CHEN, YI - ALDERSON-BROADDUS COLL; SHUFRAN, KEVIN

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2004
Publication Date: 2/1/2005
Citation: Jones, D.B., Giles, K.L., Chen, Y., Shufran, K.A. 2005. Estimation of hymenopteran parasitism in cereal aphids using molecular markers. Journal of Economic Entomology. 98(1):217-221.

Interpretive Summary: Aphids, such as the greenbug, limit small grain and sorghum production. Chemical pesticides are often relied on to control aphids. However, many species of minute parasitic wasps naturally occur and kill aphids by first stinging the aphid and inserting an egg. The wasp's egg hatches into a larvae inside the aphid and eventually kills it. Parasitized aphids take on a distinctive form called a "mummy" from which the adult wasp emerges. Management decisions whether to use a chemical pesticide often consider the presence/absence of mummies. If present, chemical treatment is often not needed. Assessing the occurrence, population level and species of wasps currently depends on observing aphid mummies, collecting them, and waiting for the adult wasps to emerge. This procedure is time and labor intensive, plus the wasps are so tiny and similar looking it is very difficult for even an expert to identify them. We developed molecular genetic markers based on the polymerase chain reaction (PCR) that allows us to determine if live aphids are parasitized (and by what species) before the mummy stage. The PCR technique was shown to be very accurate in determining the species and percent of the aphid population parasitized. The results did not differ from manually collecting and rearing aphids in the laboratory, but could be accomplished in less time. Therefore, using PCR and molecular genetic markers could allow agriculturists to determine if a chemical pesticide treatment is or is not needed in a more timely fashion. The technique also benefits scientists studying the population biology of parasitic wasps and their aphid hosts.

Technical Abstract: Polymerase chain reaction (PCR) primers were designed and tested for detection and identification of immature parasitoids in small grain cereal aphids. The PCR technique was evaluated for (1) greenhouse reared greenbugs, Schizaphis graminum (Rondani) parasitized by Lysiphlebus testaceipes (Cresson) and (2) aphids collected from winter wheat fields in Caddo county Oklahoma. For greenhouse samples, parasitism frequencies for greenbugs examined by PCR at 0, 24 and 48 h after removal of L. testaceipes parasitoids, were compared to parasitism frequencies as determined by greenbug dissection. PCR was unable to detect L. testaceipes in greenbugs at 0 and 24 h post-oviposition, but was able to detect parasitoids 48h post-oviposition at frequencies that were not significantly different from dissected samples. Field collected samples were analyzed by rearing aphids from each sample, and comparing frequencies of mummies developed with samples examined by PCR. Aphid samples included corn leaf aphids, Rhopalosiphum maidis (Fitch), bird cherry-oat aphids, R. padi (L.), English grain aphids, Sitobion avenae (F.), and greenbugs. Mummies were isolated until adult emergence, where upon each parasitoid was identified to species (L. testaceipes was the only parasitoid species found). Parasitism detection frequencies for PCR were not statistically different from parasitism frequencies of reared aphids. These results indicate that PCR is a useful tool for providing accurate estimates of parasitism and for identification of immature parasitoids to species.