Spatial Pattern of the Cabbage Looper, Trichoplusia ni, on Crucifers

Counts of the cabbage looper, Trichoplusia ni (Hübner), on crucifers did not conform to the Poisson distribution, there being an excess of uninfested and highly infested plants over the expected numbers. But when observed distributions were fitted to the negative binomial series the discrepancies were not significant when tested by chi-square. The negative binomial parameter k tended to increase with density. Using a common k, the distribution of the various stages may be described by expansion of (q-px)−k, when values of k are as follows: egg 1.72, first instar 1.28, second instar 1.28, third instar 1.30, fourth instar 1.41, fifth instar 2.07, pupae 2.42. Aggregation resulted from initial nonrandom deposition of eggs. Randomness increased with development of the larvae owing to differential elimination by biological control agents, and further increased at pupation, apparently as a result of migration of mature larvae from densely occupied plants. For all stages the variance was proportional to a fractional power of the mean. Use of the halfplant as a sample unit had little effect on the skewness of the distribution. Three transformations are offered for stabilizing the variance of field counts.