RESPONSE OF A PREDATOR TO VARIATION IN PREY DENSITY AT 3 HIERARCHICALSCALES - LADY BEETLES FEEDING ON APHIDS

We investigated the response of two species of lady beetle, Coccinella 7-punctata and Hippodamia variegata, to spatial variation in the dens ity of aphids on fireweed, Epilobium angustifolium. Our approach invol ved experimental manipulations of aphid densities and field observatio ns of lady beetle foraging at three different scales: (1) individual l ady beetles on individual fireweed stems, (2) populations of lady beet les on individual fireweed stems, and (3) populations of lady beetles in populations of fireweed stems. At the finest scale, we found that i ndividual lady beetles remained on single fireweed stems longer in the presence of large aphid colonies. However, this behavioral response t o aphid density was weak, explaining only 11 and 4% of the total varia nce in the length of time C, 7-punctata and H. variegata remained on t he stems. To examine directly the population-level effects of this ind ividual lady beetle behavior, we conducted a second series of experime nts that focused on the response of populations of lady beetles; we re leased large numbers of lady beetles into field plots in which we had manipulated the number and size of aphid colonies. By sampling the dis tribution of lady beetles, we showed that the number of lady beetles f ound on fireweed stems depended significantly on the presence and size of aphid colonies. The average number of both C. 7-punctata and H. va riegata found on a stem was 10-20 times greater when the stem containe d a large aphid colony than when the stem contained no aphids. Finally , we examined the consequences of these patterns at the scale of popul ations of lady beetles and populations of fireweed in 25-m2 field plot s. We found that the number and size of aphid colonies within the plot s explained 50 and 90% of the variation among plots in the average len gth of time that C. 7-punctata and H. variegata remained. Taken togeth er, these results demonstrate that although individual lady beetle res ponse to aphid density is extremely weak, the cumulative effect of man y individuals can produce strong population-level aggregation of lady beetles in areas of high aphid density.