The Vegetarian Side of Carnivores: Use of Non-prey Food by Parasitoids and Predators

Predaceous and parasitic arthropods can play an important role in the regulation of herbivore populations. However, the majority of predators and parasitoids also use plant-derived foods as a source of nutrients. This vegetarian side of the menu may include various plant-provided substrates, such as nectar, pollen, fruits or foods indirectly derived from plants (e.g., honeydew or pycnial fluid of fungi). Predators and parasitoids may either use plant-derived food as a supplement, or they may strictly depend on these foods during part of their life. Despite the obvious importance of non-prey food, little is known about the extent to which particular categories of plant-derived foods contribute to the diet of predators and parasitoids under field conditions. To the foraging insect the potential value of a given food source will depend on its availability, detectability, accessibility and nutritional composition. Plant-provided foods can have a dramatic impact on longevity, fecundity, and distribution of predators and parasitoids. As each of these parameters affects the local number of carnivores, the availability of suitable plant-derived food can have a major impact on mass-rearing programs, as well as on herbivore-carnivore dynamics in the field. OVERVIEW OF RESOURCE USE NON-PREY FEEDING BY CARNIVOROUS ARTHROPODS Predators and parasitoids are usually identified by their carnivorous lifestyle. Due to this bias, we easily overlook the fact that the majority of these “carnivores” also require plantderived foods as a source of nutrients. The level in which predators or parasitoids depend on primary consumption varies. (Wäckers and van Rijn 2005) distinguish between the categories of ‘life-history omnivores’, ‘temporal omnivores’ or ‘permanent omnivores’. Life history omnivores include those natu__________________________________ Selecting Food Supplements for Conservation Biological Control Second International Symposium on Biological Control of Arthropods 421 ral enemies that are strictly dependent on plant-derived food during part of their life cycle, such as hoverflies and many parasitoids. Temporal omnivores supplement their carnivorous diet during part of their life (e.g., host-feeding parasitoids), whereas permanent omnivores retain an assorted diet throughout their lifecycle (e.g., predatory mites and ladybirds). WHAT’S ON THE MENU? NON-PREY FOOD ITEMS USED BY PREDATORS AND PARASITOIDS Predators and parasitoids may feed on various substrates. Their fare may include carbohydrate-rich foods such as such as floral nectar, extrafloral nectar, fruits, plant sap, gall secretions, honeydew, Lycaenid dorsal gland secretions, and fungal fluids as well as lipidor protein-rich sources such as pollen, food bodies, and elaiosomes (Wäckers 2005). In some cases predators may also feed on plant productive tissue, which would classify them as potential herbivores (Coll and Guershon 2002; Eubanks and Styrsky 2005). A few predators exploit a broad range of the above-mentioned food items. This applies especially to ants, which have been the driving force in the evolution of many food-mediated mutualisms (Beattie 1985). The majority of predators and parasitoids restrict their diet to one or a few alternative foods. Most parasitoid species are restricted to feeding on sugar-rich solutions such as nectar and honeydew. Many predators like hoverflies, lacewings, anthocorid bugs, ladybeetles, and predatory mites feed on pollen as well as nectar/honeydew (Wäckers and van Rijn 2005). EFFECTS ON LONGEVITY AND FECUNDITY Plant-provided food can have a strong effect on life-history parameters of predators and parasitoids. Temporal or permanent omnivores can use foods like (extra-) floral nectar, pollen or honeydew as an alternative to prey. This diet extension therefore allows them to bridge periods of low prey availability (Limburg and Rosenheim 2001). When combined with predation, nectar and pollen feeding can increase predator fitness over prey feeding alone (Porter 1989; van Rijn and Sabelis 2005). Life-history omnivores, on the other hand, fully depend on non-prey food, usually during their adult stage. Their longevity and fecundity are often seriously compromised in the absence of these food sources. An example of the latter category is the large category of parasitoids that do not engage in host-feeding. At the time of adult emergence, their energy reserves often cover no more than 48 hours of the individual’s energetic requirements. Sugar feeding can increase a parasitoid’s lifespan considerably; up to 20fold under laboratory conditions for several hymenopteran parastoids ( Fadamiro and Heimpel 2001; Jervis et al. 1996; Wäckers 2001), and 2-3-fold for the phorid fly, a dipteran parasitoid of imported fire ants (Chen et al. 2005; Fadamiro et al. 2005). In addition, sugar feeding can benefit a parasitoid’s fecundity, not only through an increase in reproductive lifespan, but also through a positive effect on the rate of egg maturation (Jervis et al. 1996). This means that parasitoids that fail to replenish their energy reserves through sugar feeding will suffer severe fitness consequences. DIFFERENCES IN SUITABILITY Not all potential food sources are suitable for a given predator or parasitoid. There is substantial variation between and among food categories with regard to their availability, apparency,