Lycosid spiders and alternative food: Feeding behavior and implications for biological control

The extent to which generalist arthropod predators feed on natural populations of alternative prey, and how availability of such food influences biological control of agricultural pests, is largely unknown. A major reason is that direct estimation of arthropod predation in food webs is a methodological challenge. Here, we report the first use of DNA-technology to track predation by lycosid spiders on natural populations of springtails. Predators were collected in cereals and leys on organic farms in central Sweden. In parallel, the availability of springtails and other potential alternative prey was monitored. The collected spiders (n = 469) were analyzed using group-specific springtail PCR-primers. Spiders collected in cereals had previously been screened for consumption of an aphid pest, and by combining those data with availability of alternative prey, we tested if presence of alternative food affected aphid predation. In total, 20% of Pardosa were found to have preyed upon springtails. The proportion of spiders consuming springtails did not differ between habitats; 19% positive in leys and 22% in cereals. Springtail consumption was not correlated with springtail abundance. Our results demonstrate that springtails are an important source of alternative food for Pardosa. Thus, presence of springtails may contribute to spider population maintenance and, indirectly, to enhanced predation pressure on co-occurring pests. However, we also found evidence that high abundances of alternative prey may interfere with aphid predation. We suggest that changes in the spiders´ foraging behavior, rather than increased springtail consumption, is the underlying mechanism.