Besbicus mirabilis Kinsey

Life history

AGAMIC GENERATION. The heterosexual females oviposit during late April to mid-June. Typically, the globular galls containing agamic females appear from mid-May until late August, and normally continue development for about 10 weeks. Galls that appear after mid-July seldom attain maturity. The galls form on the principal veins of the leaf, nearly always on the under surface. Before a gall becomes evident its position on the leaf is marked by the swollen egg-slit in the underside of the vein, and a pale transverse hump on the upper surface of the leaf.

The gall first appears as a tiny fuzzy green swelling that becomes spherical and changes color to a pale yellow. As development progresses the gall becomes smooth, soft yellow or green, finely spotted with reddish brown, and matures into a thin-shelled brownish-yellow globe. The galls are about 0.5 mm. in diameter when first noticeable, and their mature size ranges from approximately 5.0 mm, to 30.0 mm., with the average about 13.0 mln. Many of the galls become misshapen because of insect activities, crowding, or injury. Ordinarilly 1-4 galls may be found on a leaf, but frequently the number is much greater and up to 38 galls have been observed on a single large oak leaf. One small oak tree, 12 inches high, with 65 leaves on its 48 inches of branches, had 72 large galls (Fig. 5). Such concentrations of galls are usually found in low growth next to the ground, where the ovipositing females are afforded greater protection. On Vancouver Island the growth season for the galls is usually over by mid-September, when the oak leaves begin to fall. The galls may be blown from the leaves or fall with them to the ground and become covered with autumn litter. The galls are practically waterproof and their double wall provides excellent insulation for the young insect. The agamic females emerge from these galls from mid-December to early January, and may live as long as 6 weeks. They leave irregular subcircular exit holes in the galls. The adult (Fig. 1) and gall (figs. 3, 5) of agamic B. mirabilis have been adequately redescribed by Kinsey (1929).

There may be considerable variation in the life history pattern because the pupae and ultimate-instar larvae frequently go into diapause for periods varying from a few weeks to several months. The immediate factors influencing the diapause periods have not been ascertained. The diapause is apparently responsible for a 2-year population cycle, augmenting the regular population every other year. During the progress of this study, leaf galls were abundant in even years and relatively scarce in odd years, with the exception of 1962-1963 when there was a moderate population in both years. This cycle apparently has little effect on the numbers of associate insects, most of which-could utilize other hosts.

- David Evans: (1967) The bisexual and agamic generations of Besbicus mirabilis (Hymenoptera: Cynipidae), and their associate insects^©