Distribution of the Immature Stages of the Diamondback Moth, Plutella maculipennis (Curt.) (Lepidoptera: Plutellidae), on Cabbage

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Abstract
Many experiments in entomology are evaluated by counts of the number of individuals in each unit of space or time. The original counts may be summarized in a frequency distribution showing the number of units containing 0, 1, 2, 3 … individuals of a given species. If a population is distributed over a number of units at random, the distribution of numbers per unit will approximate a Poisson series, the variance of the population being equal to its mean. Such distributions are rare in nature. Usually there is an excess of unoccupied units and of densely occupied units over Poisson expectation. This departure from randomness is attributable to contagion, where the presence of one insect in a unit increases the probability of others occurring in that unit. It results from the egg-laying habits of adult insects, gregariousness of feeding larvae, etc.
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