The main threat carrots face is from slugs and snails during the early seedling stage. Though cockchafer larvae can also do damage. Below are some hints on how to protect your carrots from these pests. Though note that there are other pests that can also damage carrots, but I have not come across them. And as this site is largely based on my (John Ditchburn) gardening experience they are not listed here.
Cockchafer larvae are the juvenile form of the cockchafer beetle. The beetles lay their eggs in the ground. When the larvae hatch out they feed off a variety of roots, including carrot, eventually growing to a size of about three centimetres long before pupating and emerging as a beetle to repeat the cycle. While larger cockchafer larvae can do significant damage, mostly the damage is minor and does not make the affected carrot inedible.
PROTECTING CARROTS FROM COCKCHAFER LARVAE
The pesticide Baythroid will kill cockchafer larvae but it is quite toxic and is NOT suitable to be applied to vegetable beds. The best defence against cockchafer larvae is to rotate the carrot beds each year and plant carrots in small batches so that mature carrots are not in the ground for extended periods of time. The longer carrots are in the ground the more likely they are to be damaged by cockchafer larvae. But even when damage occurs the damage is usually minimal, if you apply Percentage Factor principles there is no need to actively kill cockchafer larvae other than to squash them or feed them to the chickens when you occasionally dig them up.
Larvae of the cockchafer beetle. Note that there are a number of different types of cockchafer beetles, but they have similar lifecycles and all feed off plant roots. Image courtesy of the internet.
LEFT: Damage to carrots caused by cockchafer larvae. RIGHT: With all but the most damaged carrots it is possible to peel away the damaged parts, leaving you with perfectly edible carrots.