They crawl up vegetation to attack. Seasonal (summer-fall) in temperate climates and year-round in warm regions, they create itchy wheals often followed by scaling and are a fairly common dermatosis in horses. The larvae may vary in color from red to orange to yellow (red seems the most common). They feed 7 to 10 days then the larvae drop off the host to molt. (The life cycle takes about 50 to 70 days.)
HHH's for chiggers:
Ranging from mild to severe, stress reactions within one hour of chemical worming may include:
- broadcast diatomaceous earth or dust with sulfur (1,000 ft. needs about 5 lbs) for chigger control.
- horsemint or lemonmint (Monarda citriodora) is a great chigger repellent!
Lemonmint can be ordered from your local plant store, especially if they carry herbs. Even if you don't find it on the shelf, they will probably order it if they carry herbs (although it is the end of the normal season for herbs). Once started in the pasture, lemonmint will spread year after year.
For your situation I would spread diatomaceous earth (DE) because it is not expensive, causes no harm to your horse or other animals, and can cover a large area easily. Since you board your horse and the pastures have chiggers I think it is more than fair to ask the owners to put out the DE, although if they won't it is still worth the investment to avoid itchiness and other issues that will likely occur when riding the area. The local feed store "should" carry DE.
As for catnip I have not heard of it for chiggers, although I have found it not that easy to grow nor prosper in the pasture. It doesn't have any special soil needs and likes varied sun exposure, so once started it will reseed itself each year. You could even buy seeds, start it indoors, then plant it outside. You may even find it growing wild near streams and roadsides.
If your horse is constantly bombarded with chiggers in an infested area, it is nearly impossible to keep them from crawling up plants and lodging in the moist areas. I would not let the problem go unattended; as both you and your horse are affected!