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

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

Hymenopteran parasitoids synthesize ‘honeydew-specific’ oligosaccharides.

1. Many arthropods depend on carbohydrate-rich food sources such as nectar or honeydew. Nevertheless, we often know little about the extent to which various sugar sources contribute to the diet of arthropods. 2. One way to study food use in the field is to analyse guts of collected insects for source-specific compounds. Sugar sources often show distinct differences in their carbohydrate composition. This applies especially to honeydew, the excretion product of phloem-feeding ‘Sternorrhynchae’, which often features a broad range of phloem-feeder synthesized di- and oligosaccharides. 3. Out of these oligosaccharides, melezitose, has been widely used as an indicator of honeydew consumption. The use of melezitose or other honeydew saccharides as ‘signature sugars’ hinges on the assumption that the production of these sugars is unique to honeydew-secreting insects. 4. Here we show that the hymenopteran parasitoids Diadegma semiclausum and D. insulare synthesize the trisaccharides melezitose and erlose as well as the disaccharide maltose when fed sucrose, but not when feeding on an equimolar glucose–fructose mixture. 5. The presence of melezitose, erlose and maltose was confined to the parasitoid's digestive tract, indicating that enzyme activity is restricted to this area. D. semiclausum excrement contained low overall sugar concentrations and low relative levels of melezitose, erlose and maltose. 6. Possible functions of sugar synthesis in these nectar-feeding insects are discussed