Gnorimoschema baccharisella Busck

This species is widespread in coastal California, where the larvae cause hard stem galls on Baccharis pilularis (Asteraceae).

Biology.- Gnorimoschema baccharisella is not limited to the immediate coast where the typical, prostrate form of the hostplant occurs. The moth occurs inland, probably throughout the range of the plant, which is widespread in coastal foothill areas, often as an invader in disturbed fields. The original description was based on specimens reared by Koebele probably from Berkeley and Alameda (type locality erroneously cited by Busck as Berkeley County), who found the galls abundant in June. The moth's biology and the parasitoid complex have been studied in detail by Tilden (1951). We have reared the species and/or its parasitoids at several sites and have observed the galls at many others.

There is a single annual generation. Tilden (1951) reported that eggs are deposited on peripheral branches of the perennial shrubs, and overwintering takes place as eggs. He observed that the newly hatched larva burrows into the tip of the growing terminal. The gall begins to develop at the point of the larval entry, and the hollow gall develops around the larva. Galls develop in the newly elongating stems and are evident by February. They are green, spindle-shaped and reach full size (ca. 18-36mm in length, Fig. 103) by late February or March, but larvae are still quite small and do not survive if galls are removed from the plant before June or July. By this time often are most of them are parasitized.

At maturity, the larva cuts a hole to the exterior and drops to the ground for pupation, which evidently occurs without diapause. Emergence takes place primarily in August and September. We have collection records of the nocturnal adults from mid-July (Goleta. Sta. Barbara Co.) to late November (Big Creek, Monterey Co.).

Gnorimoschema baccharisella
Coyote brush gall moth

Hosts: Baccharis pilularis

The larva bores into the apex of the growing terminal for a short distance and seals the entrance completely with frass. A hollow gall forms around the larva, beginning at the farthest point of the larval entry. The twig usually continues to grow, leaving the mature gall some distance from the tip. The gall is short, cylindrical, often spindle-shaped, and may persist for several years. Its outside wall resembles and is confluent with the bark of the stem. Mature galls are 17 to 36 mm long. Later, the larva chews an exit hole in the gall. The larva deposits frass at the apical end of the gall during most of its development. Galls are prevalent and can be found on nearly all plants in heavily infested areas. Infested twigs die and fall off, causing noticeable pruning.

Range: very limited known distribution in the San Francisco and Berkeley areas and southern Monterey County in CA

Gnorimoschema baccharisella Busck

Baccharis pilularis

Stem gall, caterpillar, summer

Gnorimoschema baccharisella
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This gelechiid moth induces hard, integral stem galls on coyote brush [Baccharis pilularis] throughout its range.

[Range: CA]