Abstract
We studied the importance of forest structure (shape, edge length and orientation) and the crop mosaic (percentage of crops in the total land cover, within 100 and 2000 m from the forests) to the dynamics of an aphidophagous hoverfly Episyrphus balteatus. Adults were collected by Malaise traps located within and on the south- and north-facing edges of 54 forests. In winter, E. balteatus was only found on south-facing edges because of the greater insolation and temperature. In summer, it was more abundant on north-facing edges because of the abundant presence of flowers. In spring, more adults were found on long and south-facing edges than on northern edges. The presence of shrubs within 2000 m also positively affected abundance. In autumn, abundance was positively associated with length of the north-facing edge and forest shape. Emergence traps revealed that in southern France, E. balteatus may overwinter in the larval or puparial stage in forest edges. Overwintering was earlier reported only in adults. Landscape structure, length of forest edges and probably presence of shrub fallows, influence abundance of Episyrphus balteatus.
Highlights
Because most of the natural enemies of crop pests do not carry out their complete life cycle in cultivated fields, many studies focus on the important role of uncultivated elements of the farmed landscape, such as hedges, field margins, “beetle banks” and fallows, in increasing their populations and in improving their efficiency (Russel, 1989; Landis et al, 2000; Gurr et al, 2004) as control agents
While the abundance of males in autumn could not be related to any explanatory variable in a Partial Least Squares (PLS) model, all other models were significant with a Q2 higher than 0.097 (Table 2)
The specimens emerged mainly in May, but ranged from the end of April to the beginning of June. Interpreting these results from the agronomic point of view of biological control of aphids, for which early arrival of a natural enemy is required in spring (HonČk, 1983; Tenhumberg & Poehling, 1995; Corbett, 1998), leads first to a focus on parameters which favour overwintering E. balteatus females
Summary
Because most of the natural enemies of crop pests do not carry out their complete life cycle in cultivated fields, many studies focus on the important role of uncultivated elements of the farmed landscape, such as hedges, field margins, “beetle banks” and fallows, in increasing their populations and in improving their efficiency (Russel, 1989; Landis et al, 2000; Gurr et al, 2004) as control agents. Patch isolation may have a positive effect on population density for prey or host species which are controlled by enemies. It may disrupt the enemy’s ability to detect the prey (Kareiva, 1987) as Kruess & Tscharntke (1994) showed with isolated fields of bush vetch (Vicia sepium) which were less colonised by parasitoids of seed feeders
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