The vertical distribution of green peach aphids and its effect on a model quantifying the relationship between green peach aphids and a predator

The vertical distribution of green peach aphids (GPA) on potato plants was studied to estimate its impact on a model simulating the effects of a predator on GPA populations. This model is based on laboratory studies of a ladybird beetle feeding on GPA that was placed exclusively on the bottom leaf of a three leaf stem section. The model predicts the impact ofColeomegilla maculata (DeGeer) adults and third instar larvae on GPA populations over a range of temperatures and GPA densities. The vertical distribution of GPA in a potato field and the effects of different levels of aldicarb on this distribution were determined by counting all GPA on the upper, middle, and lower thirds of a stem in situ. A one leaf sub-sample was taken from each stem section as a comparison. The vertical distribution of GPA changed with aldicarb dose. Most GPA were found on the lower third of stems from plots treated with the lowest aldicarb doses. However, most GPA occurred on the upper third of the stems from plots treated with the highest aldicarb doses. This distribution changed midway through the season until most GPA occurred on the bottom third of the stems. C. maculata adults and third instar larvae preferentially search the upper and lower thirds of potato plants. This suggests thatC. maculata would be an effective early season predator, and that our model should over-predict the effect ofC. maculata on GPA populations late in the season.