Chrysomelid Beetle Movements in Relation to Host-Plant Size and Surrounding Non-Host Vegetation

Abstract

The effects of host—plant size and surrounding non—host vegetation on herbivore population dynamics were examined for two species of cucurbit—feeding chrysomelid beetles. Population densities were assessed in an experimental garden where host—plant patches were (1) either fertilized or unfertilized and (2) surrounded either by large non—host plants (tomato) or by mowed grasses. In order to assess the role of movement patterns in influencing beetle population densities, a second related experiment examined the flight behavior of individual beetles. Both Acalymma vittatum (striped cucumber beetle) and Diabrotica undecimpunctata howardi (spotted cucumber beetle or southern corn rootworm) were more abundant in patches without tomato edges. D. undecimpunctata howardi was also more abundant in fertilized than in unfertilized patches, while A. vittatum was not affected by fertilization. When host—plant size was controlled for A. vittatum was still affected by the presence of tomato edges. In contrast, D. undecimpunctata howardi was no longer affected by tomato edges, but remained more abundant in fertilized patches. Thus it appears that tomato edges can have two different and important effects on insect herbivore abundance: (1) direct effects (e.g., as in A. vittatum), or (2) indirect effects resulting from changes in host—plant size or quality (e.g., as in D. undecimpunctata howardi). Mark—recapture and movement behavior experiments indicated that the two beetle species exhibited different movement patterns. A. vittatum colonized patches without tomato edges significantly more frequently than other patches, but only 24.3% of subsequent recaptures revealed interpatch movement, and beetles moving betwen patches showed no selectively with respect to patch type. In contrast, D. undecimpunctata howardi showed both initial selectivity and subsequent movement in response to differences in plant size and quality caused by the fertilization and edge treatments. D. undecimpunctata howardi colonized patches without tomato edges and with fertilization significantly more frequently than other patches. Furthermore, 38.7% of subsequent recaptures revealed interpatch movement, and moving beetles preferentially chose patches without tomato edges. Abundances of A. vittatum seemed to be largely determined by patch choice during initial colonization while for D. undecimpunctata howardi there was considerable post—colonization movement between patches that influenced beetle distribution. Results on individual flight behavior revealed that D. undecimpunctata howardi was nearly seven times as likely to initiate flight as was A. vittatum. For D. undecimpunctata howardi flights were more likely to lead to emigration from patches that were unfertilized but surrounded by tomato plants than from all other treatments. Taken together, these differences between the two species in movement behavior may help explain observed distribution patterns. The results are consistent with previous studies examining patch size and diversity effects on herbivorous insects.