A Differential Role of Volatiles from Conspecific and Heterospecific Competitors in the Selection of Oviposition Sites by the Aphidophagous Hoverfly Sphaerophoria rueppellii

The selection of oviposition sites by syrphids and other aphidophagous insects is influenced by the presence of con- and heterospecific competitors. Chemical cues play a role in this selection process, some of them being volatile semiochemicals. Yet, little is known about the identity and specificity of chemical signals that are involved in the searching behavior of these predators. In this study, we used olfactometer bioassays to explore the olfactory responses of gravid females and larvae of the syrphid Sphaerophoria rueppellii, focussing on volatiles from conspecific immature stages, as well as odors from immature stages of the competing coccinellid Adalia bipunctata. In addition, a multiple-choice oviposition experiment was conducted to study if females respond differently when they can also sense their competitors through visual or tactile cues. Results showed that volatiles from plants and aphids did not affect the behavior of second-instars, whereas adult females strongly preferred odors from aphid colonies without competitors. Odors from conspecific immature stages had a repellent effect on S. rueppellii adult females, whereas their choices were not affected by volatiles coming from immature heterospecific A. bipunctata. The results imply that the syrphid uses odors to avoid sites that are already occupied by conspecifics. They did not avoid the odor of the heterospecific competitor, although in close vicinity they were found to avoid laying eggs on leaves that had traces of the coccinellid. Apparently adult syrphids do not rely greatly on volatile semiochemicals to detect the coccinellid, but rather use other stimuli at close range (e. g., visual or non-volatile compounds) to avoid this competitor.

When defense backfires: Detrimental effect of a plant’s protective trichomes on an insect beneficial to the plant

The plant Mentzelia pumila (family Loasaceae) has leaves and stems densely covered with tiny hooked trichomes. The structures entrap and kill insects and therefore are most probably protective. But they are also maladaptive in that they incapacitate a coccinellid beetle (Hippodamia convergens) that preys upon an aphid enemy (Macrosiphum mentzeliae) of the plant. The adaptive benefit provided by the trichomes is evidently offset by a cost.

Miriamin, a defensive diterpene from the eggs of a land slug (Arion sp.)

The eggs of the land slug Arion sp. contain a diterpene, miriamin, characterized as a polyoxygenated geranylgeraniol derivative. In bioassays with a coccinellid beetle, Harmonia axyridis, miriamin was shown to be potently antifeedant, indicating that the compound plays a protective role in nature. It is suggested that mucilaginous soil-inhabiting organisms, given their intense exposure to pathogens and predators, may be a rich source of chemical defensive agents.