Now that the heat wave has given way to more seasonal temperatures, it is once again pleasant to return outside and begin some late summer tasks. However, for many it will not be the heat that will drive them back inside but itchy welts on the arms, legs and lower torso. For the past two years, many Midwestern gardeners have dealt with a nasty unseen attacker, and this year should be no different.
Here is what you need to know about oak leaf gall mites and what you should do to protect yourself if you spend any time outside this fall.
Oak leaf gall mites (Pyemotes herfsi) are a species of itch mites that first appeared in 2004 in several Midwestern states. The oak leaf gall mite is a tiny, 0.2 mm (1/125 inches) long mite that is barely visible to the naked eye. It is related to the straw itch mite, which has pestered farmers handling straw and stored products for hundreds of years. When the mite bites a human, it causes a raised, reddened, 1/2- to 3/4-inch (diameter) welt with a hard crusty vesicle in the center - itchy, but painful when scratched. The bites often result in secondary bacterial infections. The mites must be in contact with a person's skin for four to five hours for the bite to be felt 10 to 16 hours later. Because of the delayed reaction, many people do not know they have been bitten until the next day.
The mite's life cycle is unusual and goes something like this: A mated female searches for a host on which to feed. She is small enough to be carried by the wind, so she gets blown from tree to tree in search of a leaf marginal roll gall on an oak tree. When found, she enters the gall. If she finds the tiny midge larva responsible for the gall formation, she will feed on it. Within minutes, a neurotoxin in her saliva paralyzes the midge larva, causing it to die of starvation. The toxin is potent. One bite can kill an insect 170,000 times its own weight. That same toxin is what causes itching when she bites us. Once the female starts to feed, she develops up to 250 offspring. In seven days, her progeny - of which 5 to 10 percent are males - are ready to emerge as fully developed adults.
Males emerge ahead of the females, mate with the females as they emerge from the mother and die shortly after. The females complete the cycle by dispersing in search of new hosts.
For the next several weeks, the midge larvae will be dropping out of the galls on the oak leaves to prepare for winter hibernation in the ground. However, the mites will remain active and hungry until late October into November. Raking leaves of pin and red oak trees creates the biggest risk of being bit by the mites.
Currently there are not good recommendations for a solution to this problem. Previously, the use of DEET as a repellent was recommended. But there have been several reports indicating this product was failing in giving any protection. So currently, all we can say is avoid spending time outdoors in areas infested with these mites for more than three hours at a time. Then take a shower with warm, soapy water. Do not forget to wash the clothes worn outside as soon as possible to kill any mites still on them.