I don't think DEET helps with chiggers unless you spray it on your underwear in the morning while getting dressed.

Concentrations
DEET is often sold and used in concentrations up to 100%. Consumer Reports found a direct correlation between DEET concentration and hours of protection against insect bites. 100% DEET was found to offer up to 12 hours of protection while several lower concentration DEET formulations (20%-34%) offered 3-6 hours of protection.

Effects on health

DEET is the most common active ingredient in insect repellents. Studies into the health effects of using DEET in the approved way (as a topical application) have not shown any significant harm to human health; exposure to large amounts (by means such as accidental ingestion) has similar effects to comparable exposure to other pesticides.[citation needed] As DEET is a powerful pesticide, however, manufacturers of DEET based products advise they should not be used under clothing, on damaged skin, and that the DEET based preparation be washed off after it is no longer needed or between applications.[1] In rare cases, it may cause skin reactions.[1]

Recent studies by Duke University researchers have found that "laboratory rats treated with an average human dose of DEET (40 mg/kg body weight) performed far worse when challenged with physical tasks requiring muscle control, strength and coordination. These findings are consistent with reported human symptoms following DEET's use by the military in the Persian Gulf War." [2] Duke Pharmacologist Mohamed Abou-Donia, PhD, has said that the results of this study warrant further testing and he has warned the public to use insecticides containing DEET sparingly and infrequently.[2]

Some newer repellents offer alternatives to DEET. Repellents containing picaridin or lemon eucalyptus oil were reported by Consumer Reports to be about as effective as repellents containing DEET,[3] though medical studies have differed on the effectiveness of alternatives. A number of tests have shown DEET to be the most effective and longest-lasting insect repellent available.[4]

The American Academy of Pediatrics found no difference in safety for children, between products containing 10% and 30% DEET, when used as directed, but recommends that DEET not be used on infants less than two months old.

Picaridin seems to work really well, although I haven't been out in full blackfly season. It doesn't eat plastic which is the big bonus, I don't mind that it doesn't last as long since I'm not worried about melting things when I reapply it.

I've tried other products, eucalyptus oil made me smell nice but didn't work more than 30 minutes. Other products were near worthless... well at least in the woods, they may work fine sitting on the back porch in the suburbs.