Thanks, Pete!
Fungus gnats can eat the roots of young seedlings and kill or weaken them. The larvae live in the soil and when they emerge they fly around and can spread fungal diseases between young seedlings. Consider using Mosquito-bits in your potting mix and as soon as seedlings emerge keep them in a well ventilated place.
(Diana, do you have any other suggestions regarding Fungus Gnats OR Damping off?)
More information including some control methods here (This is directed mostly at homeowners rather than gardeners!)
snip..."The most popular type of gnats are fungus gnats, which measure between 2-3 mm in length and are usually found around windows. The larvae of gnats mainly feed on fungi and decaying plant material but will eat through healthy plant roots."...snip
Fungus gnats are not the same as Fruit flies, but they are both pests.
"Fruit Flies are extremely small flies that feed on sugary and fermented substances and are seen throughout the world. They breed in moist areas and can lay up to 500 eggs at a time. A fruit fly is the stereotypical unwanted visitor, that loves to visit all year round.
You never just see one fruit fly in your house, it’s always as if a whole family, with all their distant cousins, came to visit – and believe me, we’re not in the mood for that sort of company.
The reason – once one fruit fly appears, that one fruit fly alone quickly finds a surface to lay eggs, and within 30 hours, can hatch up to 500 eggs. As bone-chilling as it sounds, fruit flies hold truth to the meaning of mi casa, es su casa, and firmly believe that your food, is their food. "