The potential of the parasitoid Aphidius rhopalosiphi for controlling cereal aphids was tested in 16 m2 field cage experiments in 1998 and 1999. In the first year, aphids and parasitoids were released in cages containing naturally occurring populations of aphids and their natural enemies. In the second year, aphids and parasitoids were released in cages which had been cleared of insects by applying insecticide. The growths of the aphid populations in the different cages were analysed and compared. In 1998, the release of 50 pairs of parasitoids per cage had no significant effect on aphid population growth relative to that in the control cages. Even though the aphid population growth rates were less than 60% of that in the control cages, in the cages in which 100 pairs and 200 pairs of parasitoids were released, it was not possible to show they statistically differed. The aphid populations in these three cages were held below 10 aphids per tiller. In 1999, the aphid density was higher and the population grew faster than in 1998. The release of 100 and 200 parasitoids per cage significantly reduced aphid population growth. A. rhopalosiphi seemed to be a good control agent in field cages, provided they were released at the beginning of aphid population growth.
Markers are essential to study movements of insects in their natural habitat. Among the available techniques, trace elements may be applied to insects as small as parasitoids. Rubidium is the most common element used for marking insects. In this study, we propose a simple marking technique for Aphidius rhopalosiphi De Stefani Perez with Rb, when reared on the grain aphid Sitobion avenae Fabricius (Hemiptera: Aphididae) marked on a RbCl incorporated diet. Our results show that the rubidium in an artificial diet is transferred to the aphid and eventually to the parasitoid. The content in rubidium marking did not differ between genders. The aphids stung by a marked parasitoid could not be distinguished from unmarked aphids on the basis of their rubidium content. There were no effects of rubidium on size, fecundity, longevity and sex ratio of the parasitoid, but the marked individuals emerged significantly later than the unmarked. We did not detect differences concerning host acceptance by marked and unmarked parasitoids. This technique may be applicable to other aphidophagous insects after some preliminary evaluations.
The tritrophic interactions between two different plant-host complexes, Avena sativa-Sitobion avenae, Triticum aestivum-S. avenae and the aphid parasitoid Aphidius rhopalosiphi were studied with respect to odour learning and recognition by the parasitoid. The orientation behaviour of females towards odours from either uninfested or aphid-infested oat or wheat plants was tested in a series of dual choice Y-tube olfactometer experiments. Female parasitoids had the opportunity to gain a single oviposition experience on either the oat-S. avenae or wheat-S. avenae complex before the experiment. In the first set of experiments, where A. rhopalosiphi was reared on the oat-S. avenae complex, eight odour-bait combinations were tested. The females did not discriminate between uninfested oat and wheat. After oat complex experience, females responded to odours from the oat complex, but not to odours from the wheat complex. Consequently, in a direct comparison the oat complex was preferred over the wheat complex. After wheat complex experience, the parasitoid's orientation responses gave a different picture. Both, the wheat complex and the oat complex, were then shown to be equally attractive. Hence, in direct comparison no preference was recorded between the oat and wheat complexes. In a second set of experiments, where A. rhopalosiphi was reared on the wheat- S. avenae complex, a possible influence of any pre-adult or emergence-related host plant experience could be excluded as the same results were obtained as before. At first glance the responses towards the different odour baits seem inconsistent. However, the results may be explained using a simple model with two key odour components.