Oak leaf Gall Mite

Oak leaf Gall Mite (Pyemotes herfsi) about 0.2mm in diameter, these tiny mites attach themselves to humans and feed. These mites reproduce by paralyzing the larva of other pests and feeding upon them until the female hatches up to 250 adult mites.

This gall appears on pin oak as an approximately 1/8-inch-wide lapping over of the leaf underside onto its upperside. It appears on the inside curves of the leafs lobes and is typically about 1/2 to 1 inch in length. The gall is green and not obvious during most of the growing season, being noticeable only by the lighter green leaf underside along the edge of its upperside.

At this time of year, the galls have turned brown, making them much more obvious against the green leaf uppersides. The fold is easily opened by prying back the folded leaf margin. The oak marginal fold gall is caused by a gall midge, a very small fly. Through much of the growing season, one can find the transparent to white, spindle-shaped fly larvae inside the galls.

Life Cycle :The adult males emerge from the abdomen of their mother in advance of the females and mate with the adult females as they then emerge. The ratio of males to females is estimated to be 1:10 to 1:20. After mating, the males die, having never fed. The mated females then find a gall, and enter through the small openings at the ends of the folded margin of the gall.

Marginal leaf fold gall typically attacks pin oaks but may also attack other of the ‘red oak’ group of trees. The leaf gall itch mite has also been reported from stored products and grains where they feed on various stored products pests, and from the galleries of wood boring beetles in the families Scolytidae and Anobiidae. They may overwinter in these protected areas or within the leaves or leaf litter on the ground.

The fertilized female mites attack the larvae of the gall, paralyzing it with venom that can paralyze a prey that is 166,000 times the mite’s size. The mite then feeds on the prey and remains attached until her offspring emerge. The generational time span is as little as 1 week, enabling the mites to become extremely numerous in a short period of time. Leaves sampled from infested pin oaks had up to 84% of the galls infested with mites.

Damage :Most insect galls do not seriously affect the health and vigor of healthy, well-established trees and shrubs. Occasionally, a heavy gall infestation causes severe leaf or stem deformation and premature leaf drop. However, these annoyances usually do not cause long-term damage to the tree.

Leaf galls may be especially aesthetically displeasing but they do not directly kill the plant. Twig galls may cause stem dieback that could bring about the demise of small trees. Galls discovered during inspection of trees that are declining or dying are usually not the cause of the problem.

Control: Most of the bites from Pyemotes herfsi tend to occur in the late summer into fall when the mite has built up large populations. Studies have shown that the mites can fall from trees in numbers of up to 370,000 per day. They are also easily carried by the wind and can potentially enter through window screens and thereby bite people who do not often go outdoors. Most bites, however, occur to individuals gardening, especially those raking infested pin oak leaves in the fall.

Control of the oak leaf itch mite is not easily accomplished. Tree sprays do not penetrate the galls and therefore the mites are protected. There are mixed results from the use of a mosquito and tick repellent. People can best protect themselves by limiting their time from under infested trees and by immediately removing and laundering clothing and then showering.

Physicians suggest the use of calamine lotions and other itch creams to reduce the itching that at times can be intense. Scratching of the bite locations is discouraged as it can result in secondary bacterial infections.

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