APHID-ANT INTERACTION REDUCES CHRYSOMELID HERBIVORY IN A COTTONWOOD HYBRID ZONE

In a cottonwood (Populus) hybrid zone, Chaitophorus aphids attract aph id-tending ants which subsequently reduce herbivory by the leaf-feedin g beetle, Chrysomela confluens. Observations and experimental manipula tions of aphids and beetle larvae on immature cottonwood trees demonst rated that: 1) via their recruitment of ants, aphids reduced numbers o f beetle eggs and larvae on the host; 2) these interactions occurred w ithin a few days of the host being colonized by aphids; and 3) althoug h aphid colonies were ephemeral, their presence resulted in a 2-fold r eduction in beetle herbivory. The aphid-ant interaction is most import ant in the hybrid zone where 93% of the beetle population is concentra ted (for reasons unrelated to aphids and ants). Because beetle defolia tion of immature trees is high (ca. 25%), the indirect effect of aphid s in reducing herbivory is likely more beneficial to trees in the hybr id zone than in adjacent pure zones where beetle herbivory is virtuall y absent. Tree genotype likely affects the impact of the aphid-ant int eraction on trees within the hybrid zone, since levels of herbivory di ffer between sympatric Fremont and hybrid cottonwoods.