To identify factors affecting the spatial dynamics of Trichoplusia ni (Hübner) on cabbage, the movements of individual larvae were monitored. Larvae were marked with 32p and released individually, one per plant, on cabbage plants in research plots. Their movements were monitored by recording their position on the plant daily, locating them with a Geiger counter. In the same plots, hourly measurements of temperature and relative humidity were recorded in three different parts of the crop canopy. Transition probabilities for the larvae from and to each of five vertical plant strata were modeled as definite integrals of the Beta probability density function (pdf). The shape parameters for these Beta pdf's were modeled as a function of microclimate, and the necessary parameters to do so were estimated by the method of maximum likelihood. This model predicted the data well and was more efficient than logistic regression. The model predicts that under high temperatures and low vapor pressure deficit (vpd), larvae move down to the shaded, cooler, lower parts of the plant; whereas under low temperatures and high vpd, they moved upward toward the economically important wrapper leaves and cabbage head
