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Using superparasitism by a stem borer parasitoid to infer a host refuge
Author(s) -
Edwards Owain R.,
Hopper Keith R.
Publication year - 1999
Publication title -
ecological entomology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.865
H-Index - 81
eISSN - 1365-2311
pISSN - 0307-6946
DOI - 10.1046/j.1365-2311.1999.00165.x
Subject(s) - biology , parasitoid , ostrinia furnacalis , host (biology) , ostrinia , cingulum (brain) , braconidae , lepidoptera genitalia , zoology , biological dispersal , pyralidae , ecology , population , medicine , fractional anisotropy , demography , radiology , sociology , magnetic resonance imaging , white matter
. 1. Macrocentrus cingulum Reinhard (Hymenoptera: Braconidae), a parasitoid of the European corn borer, Ostrinia nubilalis , and the Asian corn borer, O. furnacalis (Lepidoptera: Pyralidae), has high fecundity but has been reported to parasitize a low proportion of host larvae. This was corroborated in field collections: in Hebei (China) and Delaware (U.S.A.), M. cingulum parasitized only 15 and 25%, respectively, of hosts collected. 2. Because M. cingulum females cannot oviposit through plant tissue, they must parasitize hosts either before they have bored into stalks or while they are near entrance holes, so that at any one time, many Ostrinia larvae may be unavailable to M. cingulum . This refuge, together with fluctuations in abundance of foraging M. cingulum females, may explain why M. cingulum parasitizes relatively few Ostrinia larvae. 3. To test this hypothesis, levels of superparasitism were measured in the field. Low parasitism resulting from a refuge for host larvae should cause high rates of superparasitism in hosts outside the refuge. 4. Because M. cingulum is polyembryonic, the number of parasitoids per host does not indicate the level of superparasitism. Random amplified polymorphic DNA markers were used to determine the number of different genotypes emerging from each host. The resulting frequency distributions were fitted to those expected under random oviposition to estimate the proportion of Ostrinia larvae unavailable to M. cingulum . 5. In the samples from Hebei and Delaware, the level of superparasitism was much higher than expected by chance if all hosts were available. Fitting the frequencies of genotypes per host to a Poisson distribution, the authors estimated that 74–82% and 69–74% of host larvae were unavailable to M. cingulum in these collections, respectively. This means that M. cingulum parasitized 60–84% and 82–95% of available hosts in these collections, respectively. These levels of parasitism contrast strongly with the 15–25% found when all hosts were assumed available for oviposition. 6. Genetic distances of M. cingulum within and between hosts did not differ, allowing rejection of the hypothesis that high levels of superparasitism resulted from a female laying several eggs in the same host. 7. The hypothesis that M. cingulum parasitizes few Ostrinia larvae because many larvae are in a refuge explains these data and previously published information better than other hypotheses that have been suggested.