Foraging decision of aphidophagous predators and conservation biological control: A case study with syrphid antagonists

Abstract

The foraging behaviour of aphidophagous predators plays a key role in conservation biological control. However for understanding the foraging behaviour of syrphids most investigations have focussed on prey-predator interactions using larvae while overlooking the important aspect of foraging of adult hoverflies. The larvae of the hoverfly Episyrphus balteatus (DeGeer) (Diptera: Syrphidae) are aphid predators while the adults depend on pollen and nectar for reproduction and longevity, hence they depend on availability of convenient flowering plants. However, in agroecosystems, the spatial and temporal patterns of crops as well as accompanying flowering plants change. This leads to a fluctuation in food resources which may limit syrphid predatory performance. Thus it is important to understand how food utilization affects the fitness of adult hoverflies and how this affects conservation biological control. In the first part of the thesis the impact of resource availability on the fitness of E. balteatus was investigated using oilseed rape plants infested with Brevicoryne brassicae L. (Hemiptera: Aphididae) while pollen and sugar supply was manipulated to simulate limitation in food resources. Oviposition was evaluated while foraging patterns were video recorded. During preoviposition period a lack of pollen and sugar feeding alone led to a complete absence of egg laying. Episyrphus balteatus lived 2 fold longer when pollen and sugar were available throughout compared to feeding on sugar only. Feeding on pollen alone resulted in similar longevity as when feeding on pollen and sugar throughout. Female E. balteatus visited food patches more frequently during pre-oviposition period than during oviposition period. A similar trend was observed for the residence time on food patches during both periods. These results demonstrate that pollen availability is not only essential for oviposition but also for enhanced survival. In the second part of this study, the effects of pollen feeding frequency on the fitness of E. balteatus were investigated. Pollen feeding intervals were varied from 10 to 60 minutes per day, every 2nd, 3rd, 5th day and throughout as control. A broad bean (Vicia faba. L) infested with Aphis faba Scopoli (Hemiptera: Aphididae) was used to stimulate oviposition. From all the frequencies tested the control and pollen availability every 2nd day resulted in the shortest pre-oviposition period, the highest fecundity and the longest oviposition duration. Hoverflies in those treatments lived longest and had the highest dry weight. On the contrary, the lowest longevity was with pollen every 5th day. These results demonstrate the importance of daily access to pollen or every 2nd day feeding. In the third part, the influence of selected flowering plants on the fitness of E. balteatus was determined. Based on previous studies, seven flower species were chosen: phacelia (Phacelia tanacetifolia, Hydrophyllaceae), buckwheat (Fagopyrum esculentum, Polygonaceae), cornflower (Centaurea cyanus, Asteraceae), chamomile (Matricaria recutita, Asteraceae), white mustard (Sinapis alba, Brassicaceae), nettle (Urtica pilulifera, Urticaceae) and perennial ryegrass (Lolium perenne, Poaceae). A broad bean plant infested with A. fabae was used to stimulate oviposition. Among the plants tested phacelia led to the shortest pre-oviposition period and highest fecundity. The longevity of hoverflies was enhanced the most by cornflower and buckwheat. The size of female hoverflies was not influenced by the different flower species, whereas feeding on phacelia led to the highest dry weight. An analysis of pollen protein revealed that phacelia had the highest protein content being 3 fold more than in nettle. In conclusion, this study has demonstrated that lack of pollen sources during pre-oviposition has a negative impact on egg output and longevity. Furthermore syrphids benefit from daily or at least every second day access to pollen resources to ensure maximum fitness. In improving biological control, the use of phacelia, buckwheat and cornflower to provide food resources is highly recommended for enhanced predatory efficiency.

Key words: foraging behaviour, hoverflies, fitness, biological control.