Chiggers are minuscule orange mites (related to spiders and ticks). The larvae crawl inside human skin and excrete enzymes to digest the skin of the host. Common belief is that chiggers burrow under the skin. They do not. Attaching themselves to the surface of the skin, they send out a feeding tube and imbibe the digested skin. These enzymes are what is responsible for the intense itching.

What we know about chigger larva:
Measures about 120th of an inch in size
Like thick darker wooded areas with dense leaves, bushes, dead decaying wood and bark and tall grass.
Larva is the active ingredient in chigger bites.
Chiggers like warm, moist snug places to bite.
Chiggers are not bloodsuckers.
Chiggers do not bore into the skin.
Chiggers are lazy.
Chiggers travel fast.

Chiggers like to hang out in colonies. They also like to hang out where tall grass and bushes are thick. When hiking stick to trails and avoid wading into areas where grass is tall and plentiful. When selecting a camp site, look for a place clear of brush and grass as much as possible. Sunshine should be plentiful as should the free movement of air.

For outside treatment: You can also use sulfur dust (aka flowers of sulfur). Use the recommended rate. The action here is that sulfur dioxide (SO2) is formed by oxidation in the presence of sunlight and kills the pest. The sulfur will also kill other insects, possibly the beneficials as well.

If you suspect exposure, we have had much success by taking a shower/bath and afterwards, before leaving the shower stall/tub, we apply vinegar (white or apple cider) all over our body, especially along sock and underwear elastic areas. This seems to kill the chigger before they attach. 

Valerie Worwoods suggests:
10 drops Thyme ct linalool
5 mL (1 tsp) "any" alcohol (we prefer Vodka, as we feel it is a safer alcohol than rubbing alcohol)
Apply to chigger bites (inflammation sites) every three hours.

Testimonies
1. We live in Central Texas where the chiggers are always bad, but this year tops all I’ve ever seen! We’ve tried Peppermint and Purify with little satisfaction. Cleansing is the blend that everyone likes the best, and some even believe it works as a preventative. After spending some time outside in the garden or grass, I take a shower and then spray myself down with white vinegar (which is an excellent way to kill the chiggers before they attatch and do their damage). Then if any are detected, I reach for the Cleansing.

2. I have camped in the New Jersey Pine Barrens and had great success with Geranium to repel ticks and chiggers. I literally doused myself twice daily with the oil. I also find the most relief from itchy chiggers and mosquito bites using Peppermint.

3. We live in Central Texas and use No Bites! to prevent chiggar bites. Then, after exposure, we shower and apply vinegar (as described above) and spot treat any that get missed with Peppermint. This has proven to be very successful for us. - Linda

4. I made up a simple Bug Bite Relief formula and shared it with our grandchildren in a 2% dilution with fractionated coconut oil in a roll-on applicator. They rave about how well it works for their mosquito and chigger bites. The basic formula is:
24 parts Geranium
20 parts Peppermint
14 parts Eucalyptus globulus or radiata
10 parts Helichrysum (Lavender is a good alternative)

4. My family went to a neighbor's place to help build a chicken run, and I applied No Bites! to myself and our youngest child (age 10). None of the others felt the need to apply it, as there didn't seem to be a mosquito issue. What we didn't realize was that the place was loaded with chiggers! The only ones who didn't have chigger bites were our youngest child and myself. I didn't expect No Bites! to be effective against chiggers, but after seeing how bad the chigger bites were on the others, I'm convinced it protected the two of us. Whew! - Linda

Reference
Worwood, Valerie Ann, The Complete Book of Essential Oils and Aromatherapy, New World Library, 2016, page 132.