Antipredator response of aphids to ladybeetles: Effect of intercropping on aphid dispersal

Dispersal of viruses is intimately tied to their vectors. Aphids are known to invest in costly antipredator behavior when perceiving cues of predators. It is hypothesized that the absconding behavior of aphids in the presence of predators can increase virus spread in fields. Whereas most of the studies investigating this hypothesis were conducted in monoculture, we studied aphid antipredator behavior in intercropping with wheat (Triticum aestivum L.)-broad bean (Vicia faba L.) as a model. The bird cherry-oat aphid, Rhopalosiphum padi Linnaeus (Hemiptera: Aphididae), is an important vector of the barley yellow dwarf virus. The effects of two natural aphid enemies, adults and larvae of the seven-spot ladybeetle, Coccinella septempunctata Linneaus (Coleoptera: Coccinellidae), on R. padi dispersion was studied under laboratory conditions. Trays composed of 7 × 8 lines of plants were considered. In intercropping, one line of broadbean succeeded one line of wheat. Six treatments were compared: in both wheat monoculture and intercropping, aphids were introduced alone, with ladybeetle larvae or with ladybeetle adults. Aphids and predators were introduced on wheat tillers in the middle of the system (source line) and aphids were counted on every plant after two and 24 hours. Results show that the total number of aphids was higher in intercropping than monoculture in treatments without ladybeetles, while the contrary was observed in the presence of ladybeetle larvae. But after 24 hours, such differences were not observed anymore. However, in receptor lines (other lines than the source one), two hours after the experiment started, aphids were more abundant in monoculture than intercropping in the presence of ladybeetle adults and larvae and after 24 hours, it was still the case in the presence of predatory larvae. These results might be explained by the non-host plant chemical cues and the physical barrier that was broad-bean plants confusing R. padi when searching for their host plants after being dropped from wheat by predators (i.e. associational resistance). This study shows that intercropping can reduce the dispersal of aphids in the presence of predators, in fine potentially limiting virus dispersal, especially shortly after aphids colonize plants.