Allison speaking. The relationship between aphids and parasitoid wasps has been of a lot of interest to us here, as the wasps that kill weevils by using them as hosts for their young to eat (from the inside out!) may benefit from having aphids around for the honeydew they provide. It can be an important sugary food source to many bees, ants and wasps in the order Hymenoptera. Another hypothesis we have is that flower nectar may also play an important role in a parasitic wasp’s diet. But how important to successful weevil parasitism is wasp nutrition? Would the size of aphid populations have any effect on the size of weevil populations through this indirect relationship?
We do know from a study that hungry wasps forage instead of searching for hosts to parasitize with their eggs. Moreover, well-nourished wasps live longer and therefore have time to produce more offspring (and therefore kill more weevils) (Jacob and Evans 2001).
Beyond that, we do not have a lot of data on what the important environmental factors (like flowers) and community factors (like aphids) for abundant parasitoid wasp populations in Wyoming alfalfa are …..yet. It is actually another member Hymenoptera that is most famous –and more extensively researched— for its honeydew collecting behavior – ants.
Aphids actually have specific adaptations to their behavior, their body shape, and even sometimes their lifecycle to coexist with ants (Way 1963). Ants also occasionally have specific adaptations. One example is the honey pot ant whose abdomen can expand to store aphid honey dew to feed the colony in times of drought.
Ants clearly benefit from this relationship— for some species honey-dew forms 60-99% of an ant colony’s diet—but what’s in it for the aphids?
As it turns out, lots of things:
- Ants defend aphids from predators like ladybeetles, syrphid flies and parasitic wasps who use aphids as hosts.
- Ants bring overwintering aphids into their nests, protecting them from cold, damp winter weather.
- In one species of ant, a female leaving the colony to start another colony elsewhere actually brings a fertilized aphid with them in her jaws. This allows aphids in turn to start a new colony of her own.
Some of these mutualistic relationships are so specific that ant and aphid species are dependent on one another. Across the board, the presence of ants increases aphids’ rate of reproduction and lowers their rate of mortality through a mix of these services; although, an aphid or two has been known to become a snack for an ant seeking a little extra protein in their diet in the process.
Way, M.J. (1963) Mutualism between ants and honeydew-producing homoptera. Annual Review of Entomology 8: 307-44
Spafford Jacob, H. and Evans, E.W. (2001) Influence of food deprivation on foraging decisions of the parasitoid Bathyplectes curculionis (Hymenoptera: Ichneumonidae). Annals of the Entomological Society of America 94: 605-11.