Natural enemies of onion thrips (Thrips tabaci Lindeman) in New York onion agroecosystems

Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is a major insect pest of dry bulb onions (Allium cepa) in New York.  Recently, growers have shifted toward novel selective insecticides for thrips control, suggesting compatibility with natural enemies.  Little is known about the natural enemy profile and their potential for thrips control in large vegetable-producing muck regions (low habitat diversity) and small-diversified vegetable farms (high habitat diversity) in New York.  To assess and compare populations in both habitat types, surveys of natural enemies and onion thrips were conducted in 2011 and 2012.  Small plots of onions were transplanted at each location; natural enemies and onion thrips were recorded weekly in each plot.  The most commonly encountered natural enemies on all farms included predatory thrips (Aeolothripidae), hoverfly larvae (Syrphidae), ladybird beetles (Coccinellidae), lacewing larvae (Chrysopidae), and minute pirate bugs (Anthocoridae).  In 2011, natural enemy abundance was similar across all farms and did not change through time (P>0.05).  In 2012, more natural enemies were found on high habitat diversity farms than low diversity ones (P=0.0386), and more were observed in mid-July (P<0.001).  Onion thrips densities did not differ between habitat types in 2011 or 2012 (P>0.05), but populations were greatest in mid-July of both years (P=0.003).  There was no statistically significant correlation between onion thrips and natural enemy densities in either habitat type in either year.  These results indicate that natural enemies in onion systems respond to factors in addition to thrips populations, such as availability of alternative prey and other resources.