Aphids feed on phloem that is rich in sugars but poor in amino acids. As a result, aphids must feed continuously to ingest phloem in large amounts and then excrete excess sugars in the form of honeydew. Natural enemies of aphids are known to use honeydew as part of their diet and as a cue in host/prey location because honeydew typically accumulates in the vicinity of aphid populations.
Ladybird beetles are major predators of aphids since both the adult and larval stages of ladybird beetles eat aphids and some species are commonly used as biological control agents to control aphid populations. Unfortunately, up till now few studies have evaluated the effect of honeydew on the foraging behaviour of ladybird larvae on aphids, but preliminary studies evidenced that Coccinella septempunctata larvae stayed longer in areas containing honeydew of prey that was easy to catch and so more profitable. If so,does honeydew on the ground act as a prey-associated cue? does honeydew from low-quality aphid species act as a deterrent?
A recent paper published by Purandare and Tenhumberg reported interesting results about laboratory experiments carried out to explore whether honeydew accumulated on the ground is used as a foraging cue. The study also investigated whether, if honeydew is a foraging cue, larvae show differential responses to honeydew of high-quality prey Acyrthosiphon pisum Harris compared with that of low-quality prey Aphis fabae Scopoli (both: Homoptera: Aphididae) using the ladybird Hippodamia convergens (in the photo from the University of Kentucky) as a prey.
H. convergens larvae stayed longer in areas containing honeydew but did not engage in longer bouts of searching. Furthermore, larvae did not distinguish between honeydew from high- and low-quality aphid prey. As a whole, Purandare and Tenhumberg experiments assessed that H. convergens larvae are not more likely to climb a stick or plant in the presence of either A. pisum or A. fabae honeydew and nor do they seem to distinguish between the honeydew of aphids that differ in profitability.
Despite its applicative interest, honeydew on the ground may not be a reliable indicator of aphid density on surrounding plants, and the usefulness of cues to increase foraging efficiency depends on how reliable cues are. If the benefit of using honeydew as a cue is small, it is possible that not all aphidophagous predator species have evolved a response to honeydew. Moroever, it is possible that under field conditions, honeydew evaporates quickly, is washed away by rain, or that the volatile components of honeydew lose kairomonal activity in a short time. For instance, the kairomonal activity of the aphid Brevicoryne brassicae honeydew has been reported to decrease over time and to be lost completely after 72 h.
The effectiveness of spraying sugar solutions on crop fields to attract and retain natural aphid enemies is therefore at present quite controversial and could be effective with few ladybird species only.
Purandare, S., & Tenhumberg, B. (2012). Influence of aphid honeydew on the foraging behaviour of Hippodamia convergens larvae. Ecological Entomology, 37 (3), 184-192.
