1. The moth—Cydia nigricana—whose larvae bore into the pods of developing peas and render most of the contained seeds unfit for human consumption is considered by some authorities to be one of the principal insect pests of agriculture in Canada. It is particularly destructive in the Maritime Provinces and British Columbia, while in Ontario it increased to such proportions that the farmers of that province had to give up growing mid-season peas altogether.2. This state of affairs is attributed to the fact that the pea moth was accidentally introduced from Europe in 1893 without the insect parasites which attack and check it in its native home. At any rate no parasites have emerged from the representative collections of pea moth material made by the Canadian entomologists in the affected areas, while three species with a combined parasitism of up to 60 per cent. have been reared by the writer from cocoons of the moth in England. Furthermore, in Canada, where parasites of this particular pest are absent, 10–50 per cent. of the pea crop and sometimes as much as 75 per cent. or more, is said to be destroyed annually by the moth larvae, whereas in England, where they are present, the attack is usually comparatively slight.3. The paper opens with a general account of the biology of the pea moth including systematic descriptions of the adult and developmental stages, and notes on the host-plants and distribution of the insect.