Please see the following health advisory from the Sunapee Fire Department
Watch out for the hickory tussock moth caterpillar. It is a woolly, white caterpillar with black markings and long white hairs (see below).
Don't touch them if you see them! The hairs of the Hickory Tussock caterpillar, which has black tufts on its back and black spikes, can cause an allergic reaction or rash for some people who make contact with the insect. The caterpillars have microscopically barbed setae, which can cause inflammation.
This caterpillar appears between June and September and munches its way through the leaves of deciduous trees (it prefers nut-bearing trees, but will settle for willow, ash, aspen, apple, oak, and even raspberry plants and corn stalks). It has been seen in Washington lately.
The caterpillar excretes a type of chemical defense upon contact (which is more properly termed "allergenic" than "poisonous").
Most people who handle these creatures will experience a burning, nettle-type, itchy rash of mild to moderate severity, but washing the affected area with soap and water, then applying ammonia or calamine lotion and icing the area should set things to rights.
However, some people are hypersensitive to the poison and have allergic reactions to it in addition to the itchy rash, those persons are likely to experience more severe symptoms such as swelling and nausea and should seek expert medical advice as soon as possible.