Harvesting Disrupts Biological Control of Herbivores in a Short-Rotation Coppice System

Abstract

Disturbances such as harvesting often interfere with the ecological processes that lead to the biological control of insect pests. For willows, grown as short‐rotation coppice crops harvested every third to fifth year, it has been suggested that high plant quality in the resprouting shoots after harvesting may explain observed high densities of herbivorous insects, especially leaf beetles (Coleoptera: Chrysomelidae), in the plantations. In this study, we show that generalist predators may be important as regulators of leaf beetle populations. All three leaf beetle species, which were studied for five years in 12 plantations, showed a negative correlation between the population growth rate from spring to fall and the abundance of the most common generalist predator Orthotylus marginalis (Heteroptera: Miridae). For the most abundant leaf beetle, Phratora vulgatissima, we also found a significant positive correlation between its population growth rate and egg survival, indicating an overall effect of predation on herbivore population growth. Harvesting, which takes place during the winter, had a negative effect on the abundance of leaf beetles and predators. However, the first year after harvesting, all three leaf beetle species regained this loss with a very high population growth rate. A reason for the better ability of the herbivores to recover from the disturbance may be that they, unlike the predators, mainly overwinter outside the plantations. All three leaf beetles peaked in density three years after harvesting whereas the density of generalist natural enemies increased or leveled off during the five‐year period after harvesting. We conclude that predation by generalist predators is potentially important for population control of leaf beetles in willow coppice, but that the intermediate disturbance regime of around five years between harvests, appears to be too short to avoid disruption of biological control. Alternatives for more efficient biological control in short‐rotation coppice systems may be a longer period between harvests that enables the predators to fully respond numerically, to leave natural enemies refuges at harvest, or to harvest adjacent plantations asynchronously.

Agroenergy Crops Influence the Diversity, Biomass, and Guild Structure of Terrestrial Arthropod Communities

“…Perennial-based crops are projected to have reduced pesticide and fertilizer demands, longer rotation periods that reduce disturbance, and greater structural heterogeneity that more closely resembles native ecosystems [22,26]. Biocontrol services associated with arthropod predators [8,21] may further minimize the need for pesticides provided that conducive management practices such as asynchronous harvest of neighboring fields or increased periods between harvests are implemented to allow recolonization by beneficial arthropods after harvests [22,67]. Because fields that are 60-100% grass are equally digestible by refineries [68], integration of a forb component into production systems may become economically feasible as a means of enhancing natural biocontrol.…”

Overwintering sites of the Japanese carrion beetle Eusilpha japonica

“…Consequently, we conclude that E. japonica beetles prefer soils near trees as overwintering microhabitats. Ladybird beetles Coleomegilla maculate are known to utilize different habitats between seasons (Nault and Kennedy 2000), and willow beetles Phratora vulgatissima move into cracks and crevices before overwintering (Björkman et al 2004;Björkman and Eklund 2006). Olive weevils Dyscerus perforates overwinter on the ground surface within 50 cm of the base of tree trunks (Ichikawa et al 1991), and spider mites Neoseiulus fallacies and riparian ground beetles Bembidion petrosum move from moist to dry microhabitats before overwintering (Epstein and Kulman 1990;Ishitani and Yano 1994;Yamazaki et al 1999;Anderson 2006).…”

“…Most studies on habitat preference focus on spatial distribution and behavior during the developmental season (e.g., Luff et al 1989;Rushton et al 1991). Information on overwintering sites is limited mainly to agricultural pests (e.g., Ichikawa et al 1991;Nault and Kennedy 2000;Björkman et al 2004;Björkman and Eklund 2006). Few studies have investigated how and where common carrion beetles overwinter.…”

Food Conditioning Affects Expression of Insect Resistance in Diploid Willows (<i>Salix</i> spp.)

“…Heavy defoliation by adult beetles and larvae is reported to reduce plant biomass production by 40% [7]. High leaf beetle densities occur frequently in willow plantations [8]. The use of insecticides is not feasible due to economic and environmental issues.…”

“…The breeding for resistance will almost certainly form part of such a strategy. Adapted harvesting regimes are another part in controlling the beetles [8,9] that may also affect productivity [10].…”