Investigation of the behaviour and host relationships of Anisopteromalus Calandrae (Howard) (Hymenoptera: Pteromalidae), a parasitoid of stored grain beetles

A literature review of the biology and ecology of Anisopteromalus calandrae shows that studies have concentrated on survival and oviposition rates. The host relationships and host-searching behaviour have not been studied directly to any extent. My study was therefore designed to clarify: (a) which species of pests and parasitoids are currently present in spilled grains and grain residues in southeast Queensland, (b) which species of grain beetles A. calandrae parasitizes in the laboratory and in the field, (c) if there is preference for specific host stages of the different species tested, (d) what the daily pattern of activity of females presented with an abundance of hosts, (e) whether hosts are encountered at random or whether parasitoids are guided to their hosts by specific cues, and (f) whether searching behaviour is influenced by the presence of hosts in different proportions. Nine pest beetles were recorded in spilled samples: Cryptolestes sp., Sitophilus oryzae, S. granarius, Rhizopertha dominica, Tribolium castaneum, Oryzaephilus surinamensis, Latheticus oryzae, S. zeamais and T. molitor. Residue samples contained only seven of these; S. zeamais and T. molitor were absent. Four parasitoid species were collected from spilled samples: A. calandrae, Choetospila elegans, Cerocephala dinoderi and Cephalonomia waterstoni. The field data was subjected to correlation analysis to detect possible associations of parasitoids to the pests based on the number of insects in the samples. Correlations of parasitoid densities with the various beetle species were generally high but varied among sites. The only reasonable consistent correlation was between A. calandrae densities and densities of all beetles combined. The rate of parasitism by A. calandrae in the laboratory was highest in fourth-instar larvae and lowest in second instars. The sex ratio of progeny from fourth instars was female biased, whereas progeny from second instars A were all males. A. calandrae females, when given a choice, prefer ovipositing on Sitophilus larvae to R. dominica larvae, but when there was no choice their reproductive output was not significantly different among species. When the 3 host species were placed separately, but simultaneously, in the field there was significantly higher emergence of A. calandrae from the 2 Sitophilus species than from R. dominica. In contrast, C. elegans emergence was significantly higher from R. dominica. The laboratory choice tests and the field experiments suggest that A. calandrae is specifically adapted to the Sitophilus species. The activities of mated females in the presence of an excess of their habitual host species (S. oryzae) can be categorized: (a) walking on glass side walls of the arena, (b) drumming of grains with antennae, (c) tapping surface of grain with the ovipositor, (d) drilling through the grain, (e) oviposition, (f) host feeding, (g) resting and (h) preening. Ovipositional and host feeding activity continued at an unchanged rate throughout the day. Females spent approximately 87% of their time in oviposition and related activities. Females are able to lay eggs without first host feeding. More host larvae were killed by oviposition than by host feeding when females were exposed to fourth-instar larvae. Dissections revealed that oviposition and host feeding occur in different host individuals (non-concurrent) and larvae that are fed upon are killed (destructive host feeding). In a confined area, females responded to the presence of infested grains even before encountering them. In contrast, when exposed only to uninfested grains they spent more time on the glass cover of the arena. The speed of movement was progressively reduced with an increasing rate of weevil infestation. The number of turns made by searching females was significantly higher on uninfested grains than on infested ones. The longer time spent among grains with higher infestation levels can be a consequence of an increased intensity in the stimulus from the hosts present in the grain. The results are discussed in relation to the use of A. calandrae in the bilogical control of stored product pests in grain spillages and residual stores of grain.