Effect of vegetation density, height, and connectivity on the oviposition pattern of the leaf beetle Galeruca tanaceti

Abstract

Vegetation structure can profoundly influence patterns of abundance, distribution, and reproduction of herbivorous insects and their susceptibility to natural enemies. The three main structural traits of herbaceous vegetation are density, height, and connectivity. This study determined the herbivore response to each of these three parameters by analysing oviposition patterns in the field and studying the underlying mechanisms in laboratory bioassays. The generalist leaf beetle, Galeruca tanaceti L. (Coleoptera: Chrysomelidae), preferentially deposits its egg clutches on non-host plants such as grasses. Earlier studies revealed that oviposition within structurally complex vegetation reduces the risk of egg parasitism. Consequently, leaf beetle females should prefer patches with dense, tall, or connected vegetation for oviposition in order to increase their reproductive success. In the present study, we tested the following three hypotheses on the effect of stem density, height, and connectivity on oviposition: (1) Within habitats, the number of egg clutches in areas with high stem densities is disproportionately higher than in low-density areas. The number of egg clutches on (2) tall stems or (3) in vegetation with high connectivity is higher than expected for a random distribution. In the field, stem density and height were positively correlated with egg clutch presence. Moreover, a disproportionately high presence of egg clutches was determined in patches with high stem densities. Stem height had a positive influence on oviposition, also in a laboratory two-choice bioassay, whereas stem density and connectivity did not affect oviposition preferences in the laboratory. Therefore, stem height and, potentially, density, but not connectivity, seem to trigger oviposition site selection of the herbivore. This study made evident that certain, but not all traits of the vegetation structure can impose a strong influence on oviposition patterns of herbivorous insects. The results were finally compared with data on the movement patterns of the specialised egg parasitoid of the herbivore in comparable types of vegetation structure.

Bottom-up effects may not reach the top: the influence of ant–aphid interactions on the spread of soil disturbances through trophic chains

“…These changes in plant quality and quantity should affect the abundance of insect herbivores that feed on thistles. Larger plants may sustain higher herbivore densities or be more attractive for female's oviposition [39], and enhanced plant quality improves herbivores' development rate, survival and/or fecundity, thus increasing their abundance [40,41]. Accordingly, we found that refuse dump thistles had higher aphid density than those established on adjacent soils.…”

Connectivity counts: disentangling effects of vegetation structure elements on the searching movement of a parasitoid

“…Earlier field investigations in this parasitoid‐host system showed that host egg clutches are deposited more frequently in patches with high stem density than compared with low density patches (Meiners & Obermaier, 2004; Randlkofer et al , 2009). Moreover, the success of O. galerucivorus to parasitise eggs is reduced within patches of tall and dense vegetation on a small spatial scale in the field (r = 0.1 m ) (Obermaier et al , 2008).…”

“…1.5 mm in size) mostly walk up and down plants during their search for host eggs. In general, the probability of leaf beetle oviposition increases in patches with high stem density in the field (Meiners & Obermaier, 2004; Randlkofer et al , 2009). Earlier field studies also revealed that a dense and complex vegetation structure reduces the parasitism success of the egg parasitoid on a small spatial scale ( r = 0.1 m ) (Obermaier et al , 2008).…”

Differential susceptibility and suitability of domestic and wild apple species for a florivorous weevil and its parasitoids

“…; Randlkofer et al. ). Another factor that affects parasitism levels is the density of the respective host (Cloyd and Sadof ; Hancock et al.…”