The diamondback moth, Plutella maculipennis (Curt.), is one of three species of Lepidoptera that annually cause serious commercial damage to cabbage and related crops in Ontario. It has long been underrated as a pest of cruciferous crops, possibly because of its small size; in eastern Ontario, it is much more numerous than the other two species and ranks second in importance to the imported cabbageworm, Pieris rapae (L.) . Its biology in eastern Ontario was investigated from 1951 to 1956. The history, distribution, and synonomy of the insect and general descriptions of the stages were given earlier (Harcourt, 1956). This article presents the life-history, behaviour, and host relationships. General MethodsMost of the field work was conducted at hlerivale, Ontario, five miles south of the Central Experimental Farm, Ottawa. Although experiments were largely confined to this locality, observations were made in many other areas of eastern Ontario during the six years of the study.The immature stages were studied in plots of cultivated crucifers grown under conditions similar to those practised commercially in the area. The adults were studied in the field and in oviposition cages outdoors; flight behaviour was studied by means of a controlled-interval light trap.As the senerations overlapped, it was not possible to determine the seasonal history of the insect by recording its stages in the field. Accordingly, the seasonal history was studied bv rearing the insect in a plastic-screen cage, 6% x 9 x 7 feet, in the field beside plots of cultivated crucifers at the Central Experimental Farm in 1953 and at llilerivale in 1054 and 1955. Meteorological records showed that it did not appreciably alter the physical factors of the environment (Harcourt, 1957).The durations of the developmental stages of the insect were recorded in conjunction with seasonal history studies.

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The diamondback moth, Plutella maculipennis (Curt.), is a sporadic pest of cruciferous crops throughout Canada. It is normally held in check by a multiplicity of environmental factors, chiefly biotic; however, serious outbreaks do occur (MacNay, 1948, 1953, 1957, 1959). In eastern Ontario it has been extremely numerous since late 1951, and during the present study, 1952 – 1956, it was more abundant than the imported cabbageworm, Pieris rapae (L.), or the cabbage looper, Trichoplusia ni (Hbn.) . Two earlier papers (Harcourt, 1956, 1957) presented the history, distribution, and synonymy of the insect, giving general descriptions of the stages and many aspects of its biology in eastern Ontario. This article gives the relative abundance of its parasites and predators, and discusses certain population relationships.

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An appraisal of population processes in the Colorado potato beetle, Leptinotarsa decemlineata (Say), on its principal host plant showed that the insect has no natural agents that act in a manner that prevent it from overshooting its food supply. It is little affected by weather conditions or predators, and its only parasite, the tachinid Doryphorophaga doryphorae (Riley), is inversely density dependent in its action. Once the beetle has exploited its food resource, the larvae starve and the adults emigrate in quest of other hosts.The analysis of age interval survivals showed that populations are both regulated and disturbed by the principal key factor, adult migration, which is density dependent but overcompensating. A predictive model based on density relationships explained 94% of the variance in population trend.