This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Plum fruitlets are often attacked by the larvae of a sawfly in the same manner as the Apple. The infested fruitlets have small holes in them, and so can readily be seen. As long as the maggot is present in the fruitlet this hole is blocked up with wet brown frass. The larvae move from one fruitlet to another, and must damage a good many during their period of growth. All manner of plums, gages, and damsons are attacked when quite small. The adult Sawfly appears in April and May. The female is shiny black with yellowish-red legs, and iridescent wings, and is rather more than 1/3 in. in length. The female lays her eggs in the unopened blossoms. These hatch in a few days, and the young Sawfly larvae force their way into the embryo fruit and feed on the developing kernel. The larva inside the plum can at once be told by the number of its legs, there being six jointed legs and six pairs of prolegs and an anal pair. The colour is creamy white, but some have a pinkish tinge, and when full grown reach 1/3 in. in length. When mature they enter the soil and form a cocoon of earth, in which they remain as larvae until the following spring.
Fig. 364. - Plum Grub and Moth (Opadia (Carpocapsa) funebrana).
As this pest usually seems to start on a single tree, it is well to have all infested fruit on the tree hand-picked before the insect has time to spread. The only other time we can deal with it is when the larvae enter the soil. Probably then a good dressing of vaporite worked into the soil would destroy many of the larvae before they had spun their cocoons. (See Vol. I, p. 189.)