Deconstructing insect herbivore trivial movement in a monoculture and triculture: Limitations of emigration

Abstract

Significantly higher population densities of a squash pest beetle, Acalymma vittatum, were observed in a squash monoculture than a triculture of squash, maize, and beans. One hypothesis for such differences is that non-host plants in the triculture cause the herbivore to move more frequently resulting in higher emigration rates from the tricultures. Plant to plant movement of the beetle was studied in vegetative winter squash to evaluate if differences in components of trivial movement would generate a higher emigration rate from the triculture. Tenure times were shorter on plants in the triculture than on plants in the squash monoculture, there was no movement directionality, movement distances were greater in squash monocultures than in tricultures and were greater during July than August. In the tricultures, plant-to-plant transitions were primarily to maize from non-host plants and to beans or squash from host plants during July, but during August, movement from any plant was primarily to maize. We integrated the values of the movement components from vegetative winter squash into a simple Markov model, and found that during July, emigration was estimated to be faster from the triculture than the monoculture, supporting the hypothesis. However, in August, emigration was estimated to be slower from the triculture. In August, maize plants were tall and, by acting like a fence, greatly reduced the movement distance of beetles. The effect of the differences in emigration rate on beetle population density was greatest for small patches, indicating that emigration may be important only in small-scale production systems.

Diet specialization mediates drivers of Cucurbita herbivory in a semi-arid agroecosystem

“…Our finding of strong Cucurbita density effects on overall herbivory levels resonates with past work exploring the role of plant density on herbivory risk, but also provides new insight for an understudied species and region. Previous work has shown that striped cucumber beetles ( Acalymma vittatum ) emigrate less from patches with lower densities of Cucurbitaceae plants, leading to higher beetle densities, but that these specialist beetles are also more abundant where non- Cucurbita plants act as a barrier to emigration at field edges ( Bach 1988 ; Andow 2023 ). In our study system, field edges were extensively managed to remove vegetation to allow transportation of agronomic vehicles, therefore there were few vegetative barriers to herbivore movement at field edges.…”