Influence of landscape composition on the physiological condition of common Coccinellidae

Background/Question/Methods

Coccinellid beetles are known as important predators of crop pests in agro-ecosystems. As generalists, they feed on a number of different prey species and can switch to plant derived resources in times when prey is scare. Past research has shown that the abundance and diversity of coccinellid species in crop habitats IS influenced by the composition of the surrounding landscape. However, landscape structure may also be important determinants of fitness-related traits of coccinellids, via spatial and temporal variation in quality and quantity of food supply.

To investigate the effects of landscape diversity and composition on the physiological condition of coccinellids we sampled adult beetles of common species across a gradient of agricultural intensification in locations around Madison, Wisconsin from May till September 2011. The measured indicators of body condition included the relative content of body fat and an index for body size. Both are thought to be positively related to fitness and are affected by feeding conditions. We hypothesized that the physiological condition of coccinellids would be superior in landscapes with a low agricultural intensity and a high amount of perennial semi-natural habitat compared to intensely cultivated landscapes. 

Results/Conclusions

Five species of coccinellids were found in numbers that were high enough to be included in the analysis. Overall Coleomegilla maculata was the most abundant. Both fitness-related body traits showed some temporal variability that was included in the analysis as well as the size and energy allocation dimorphism between the sexes. The findings for body size were in some cases contrasting these for fat content but unlike the allocation of fat, the size reflects the juvenile feeding conditions. The relationship between the body condition of coccinellids and the landscape composition was mainly species specific. One striking pattern, however, was the negative effect of the relative proportion of corn as well as soy bean within 1 km surrounding on the body fat of multiple species. In contrast the fat content of C. maculata was positively affected by the amount of small grains, which may provide food in the early season when other annual crops and their associated herbivores are still absent. Furthermore the amount of body fat of C. maculata was positively correlated with open perennial habitats but negatively with forest. This is consistent with previous findings showing a greater abundance of C. maculata in grassland habitats compared to wooded areas.