oak gall wasp

Philonix fulvicollis is an abundant, very small, gall-forming wasp. It occurs in the United States from Maine to Georgia, west to North Dakota and Oklahoma. It also occurs in southern Canada from Quebec to Manitoba. It is common in Minnesota.

Philonix fulvicollis is usually identified by the galls it produces. It causes galls on several trees in the white oak group. In Minnesota, this includes white oak, swamp white oak, and bur oak.

There are two generations per year, a sexual generation that produces both males and females, and an asexual generation that produces only females. The asexual females are parthenogenetic, reproducing without the assistance of males. Both generations produce galls, but the galls are very different.

The spring (sexual generation) gall appears singly in the terminal bud of a bur oak twig. The gall is seed-like, egg shaped in outline, moderately robust, and about 1⁄16″ (2 mm) long. The larva overwinters in the gall and matures in May. When mature, the gall is dark brown with parallel longitudinal grooves (striations). It occupies the entire bud. Further stem and leaf growth is either stunted or completely suppressed. An asexual female emerges in June. The gall is deciduous and falls off the tree usually by late July.

The autumn (asexual) gall appears in July along a vein usually on the underside of a leaf. It is loosely attached. It is green at first, soon becoming grayish brown. The mature gall is thin, fleshy, globe shaped, and 3⁄16″ to 5⁄16″ (5 to 8 mm) in diameter. It is densely covered with gray, felty hairs. It contains a single cell supported by filaments. The larva overwinters in the gall. The adult sexual female usually emerges the following year in September or October, but it sometimes takes up to four years for it to emerge.

The adult sexual female is 1⁄16″ (1.9 mm) in length. The body is uniformly reddish brown. The antennae are slightly paler and they have 14 segments. The finger-like sensory appendages on the mouth (palps) are yellow. The legs are yellow. The forewing is longer than the body.

The male is 1⁄16″ (1.8 mm) in length, the body is uniformly brown, and the antennae have 15 segments.

Adult total length: 1⁄16″ (1.9 mm)

Bud gall: 1⁄16″ (2 mm)

Leaf gall: 3⁄16″ to 5⁄16″ (5 to 8 mm)

bur oak (Quercus macrocarpa)

Chapman’s oak (Quercus chapmanii)

chinkapin oak (Quercus muehlenbergii)

dwarf chinkapin oak (Quercus prinoides)

gambel oak (Quercus gambelii)

overcup oak (Quercus lyrata)

post oak (Quercus stellata)

swamp chestnut oak (Quercus michauxii)

swamp white oak (Quercus bicolor)

white oak (Quercus alba)

Sources

24, 27, 29, 30, 82, 83.

Abundant

Galls of this insect were formerly misattributed to Philonix niger. That name is not a synonym because it is properly applied to another valid species.

This species has no common name. The common name of the tribe Cynipini is oak gall wasps, and it is used here for convenience.

Luciearl
9/5/2024

Location: Fairview Twp.

3 oak galls