Seasonal occurrence of Phyllophaga species and biological studies of Phyllophage ephilida (Say) on sweet potato, Ipomoea batatas (L) Lam, in Louisiana

Abstract

The biology of Phyllophaga ephilida (Say) was studied in Louisiana with both laboratory and field experiments from 1998 through 2003. The seasonal occurrence of common adult Phyllophaga species in sweet potato growing areas was surveyed with blacklight traps. P. ephilida and P. crinita (Burmeister) were the most common species at all locations. The attractiveness of seven methyl esters of L-isoleucine and L-valine blends to P. ephilida males was evaluated in sweet potato fields in 1999 and 2000. The 100% methyl ester of L-isoleucine was significantly more attractive than all other blend ratios that were tested. The life cycle of P. ephilida in southern Louisiana was determined to be univoltine. In the laboratory, the effect of three physical factors (soil moisture, organic matter content and soil texture) on the depth of adult Phyllophaga diurnal burrowing was investigated. Soil moisture and organic matter content had a significant effect on the depth of burrowing, while soil texture did not affect burrowing depth. The feeding preference of adult P. ephilida for the foliage of eight tree species was determined in the laboratory with choice tests. Beetles exhibited a significant preference for pecan, oak and elm, with maple, and sweetgum being less preferred. Magnolia and slash pine were not fed upon at all. Pheromone traps were operated in grower fields in 2001 and 2002. Data on percent white grub injury to sweet potatoes in these fields, the proximity of traps to tree lines, and the harvest date of the crop were collected. An analysis of covariance revealed that percent white grub damaged roots was significantly affected by mean adult P. ephilida captured in pheromone traps, whereas proximity to tree lines and harvest date were not significant. The year effect was significant as were interactions between year and tree line, year and harvest date, and year and mean adult capture. The covariance model had an r-square of 0.29. A linear polynomial regression model predicting white grub damage as a function of trap catch, tree line proximity and harvest date was developed. The model had an r-square of 0.18 and estimates a damage threshold (5% root damage) of one to two P. ephilida per week in pheromone traps. The pheromone trap provides sweet potato growers with a practical means to monitor adult activity and thereby determine the peak flight period and assess the need for an insecticide application timed to coincide with beetle oviposition.