Influence of target plant density on herbivorous insect oviposition choice: Ambrosia artemisiifolia L. (Asteraceae) and Zygogramma suturalis F. (Coleoptera: Chrysomelidae)

Zygogramma suturalis F. has been introduced to Russia for the biological control of the common ragweed Ambrosia artemisiifolia L. The functional response of Z. suturalis to the density of the target plant was studied at two levels: between plants and between 0.1 m² sampling plots. The number of Z. suturalis eggs per unit of plant weight was almost independent of plant weight, indicating that a functional response between plants is absent. However, tall and intermediate height plants received more eggs per phytomass unit than short plants. Between plots, correlation analysis showed that absolute Z. suturalis egg density is positively dependent on the ragweed density. Experiments showed that the locomotor activity of ovipositing females decreased when on the ragweed, suggesting orthokinesis. The number of leaf beetle eggs per unit of ragweed weight was negatively correlated with host plant density in May‐June, when the majority of eggs are laid, but was positively correlated in July and loosely, or not significantly, correlated in August. Seasonal changes in the functional response of a herbivorous insect to host plant population density have not been reported previously. A negative functional response of a herbivorous insect to target plant density can be a serious obstacle to the biological control of weeds.