Detecting nutritional state and food source use in field-collected insects that synthesize honeydew oligosaccharides

Abstract

1. During the adult stage many arthropod species, including aphid predators and parasitoids, depend on nectar and honeydew as a source of carbohydrates. Despite the importance of sugar feeding for these organisms, we know little about their energy and nutrient provision under field conditions. 2. Here we assessed the nutritional state of adult parasitoids, hoverflies and lacewings in a Swiss winter wheat (WW) and a spring wheat (SW) field and studied the contribution of honeydew to the diet of these aphidophagous insects. The total sugar level and the glucose-fructose ratio were used as indicators for nutritional state and sugar feeding. 3. Over 76% of the collected individuals from each of the three insect groups in both fields had recently consumed carbohydrates. The average nutritional state was significantly higher in the SW field for Chrysoperla carnea and Aphidius spp. 4. Honeydew consumption by insects is commonly investigated by analyzing target insects for the presence of honeydew 'signature' sugars, such as melezitose and erlose. However, our laboratory studies show that adults of the three insect orders investigated synthesize these 'honeydew-specific' sugars after sucrose feeding. 5. As the erlose-melezitose ratio of sucrose-fed Aphidius spp. and the hoverfly Episyrphus balteatus differed clearly from the honeydew sugar profiles of wheat infesting aphids, this ratio could be used as an alternative indicator of honeydew feeding. However, this method could not be used for the lacewing C. carnea. 6. Our data show that 55% (WW) and 59% (SW) of field-collected Aphidius spp. which showed evidence of sugar feeding could be classified as having consumed honeydew within the 24-48 h before collection. Evidence of honeydew feeding by hoverflies, on the other hand, was found to be much more variable, ranging from 0% in the WW field to 44% in the SW field. 7. This study shows that the detection of honeydew consumption in field-collected insects based on honeydew oligosaccharides can be feasible even when insects synthesize these oligosaccharides themselves. © 2007 The Authors.