Field margins vs. insecticides : factors affecting the density of predators attacking Plutella xylostella

Biological control provides a tool to reach goals such as maintaining sustainable agroecosystems and decreasing the use of pesticides. Studies show that generalist predators can reduce prey populations effectively and, thus, may function as good biocontrol agents. Unfortunately, generalist predators have been exploited to a small extent in biological control and little is known about how different factors can affect their density. The diamond back moth, Plutella xylostella, is a major insect pest on cabbage throughout the world but still few studies have been conducted on possible predators and factors that affect their density. Therefore, this study focuses on (1) identifying predators of P. xylostella and (2) investigating how insecticide use and type of field margin affect the density of predators in cabbage fields. The study took place in Nicaragua in six different cabbage fields, three conventional and three semi-organic. Three methods were used to measure the density of predators weekly during six weeks; d-vac, pit-fall traps and observations. According to the results more predators were found in the field margins than in the cabbage fields. The factor that affected the density most was the type of field margin surrounding the field. Field margins with a high proportion of natural vegetation had a higher number of predators compared to prepared or newly sowed margins. The amount of insecticides used affected predator density negatively only when used very frequently. Among the three major predator groups found (Araneae, Coleoptera and Hemiptera), Hemiptera was the only one which showed any difference between the two different farm types; on semi-organic farms there was an increase in density at the end of the observation period whereas the density tended to decrease on conventional farms as shown by a significant farm type by time interaction. The results also showed that high numbers of P. xylostella were never observed when there were a high number of predators. In conclusion, my results suggest that field margins may function as refuges for arthropod predators and that these refuges can have a bigger impact on the number of predators than the amount of insecticides used. The practical implications of these results are discussed and to my knowledge, there are no similar results reported previously in the literature from this or similar systems