Precocious activity of parasitoids is a key factor for cereal aphid control. We investigated the oviposition, flight and walking capacities at low temperature of four aphid parasitoids (Aphidius rhopalosiphi, A. ervi, Praon volucre, P. gallicum) on one of their aphid hosts (Sitobion avenae). Oviposition behaviour was tested at 2, 4, 6, 8, 10, 15 and 20°C, under a photoperiod of 12 h light. Some females of A. rhopalosiphi oviposited at 6°C while the percentage of parasitization below 10°C remained low for the other species. The proportion of males decreased with temperature for A. ervi, P. volucre and P. gallicum but not for A. rhopalosiphi. For all species, flight and walking activities increased with temperature, Aphidius species being active at lower temperature than Praon species. Field captures showed that the activity threshold for A. rhopalosiphi is 12°C, 15°C for A. ervi and P. volucre, and 19.5°C for P. gallicum. These results are discussed regarding the potential of these four species for aphid control.
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.