PARENT SESSION
Oral Session #41: Agroecology and Urban Ecology. Presiding: M. Liebman.
Wednesday, August 8, 2001. 8:00 AM to 12:15 PM. Hall of Ideas L&M.
Density dependent guarding of aphids by an ant on an agricultural weed.
HARMON, JASON1, ANDOW, DAVID1, 1
ABSTRACT- Lasius neoniger tends colonies of the aphid Aphis fabae on creeping thistles in corn fields. Visual counts on 50 marked plants indicated that plants with more ants had higher per capita aphid growth and lower numbers of the dominant predators, coccinellids. This suggests ants can guard aphids from coccinellids. To test this hypothesis, we manually infested 80 plants with various aphid densities, deterred ants from half the plants with tanglefoot, and counted insects daily. We found coccinellids were less abundant on plants with ants than on plants without ants when counts of ants per aphid was high. This again indicates guarding behavior. However, when ants per aphid was low, coccinellids were as abundant or more abundant on plants with ants than on plants without ants. This indicates there is variation in the protection aphids receive from coccinellids. Behavioral observations determined that differential protection was, in part, due to ant behavior on plants with different sized aphid colonies. At low aphid densities, ants tended to patrol the aphid-infested area as well as surrounding leaves and stems. At high aphid densities, counts of ants per aphid were low, less aphid-infested area was patrolled by ants, and almost no surrounding area was patrolled. Consequently, at high aphid densities coccinellids could contact more aphids without being disrupted than they could on plants with fewer aphids.
KEY WORDS: density dependent mutualism, guarding behavior, predator-prey, indirect effect