Emerging Bot Fly Larvae
19 June 2020
Spotted in Horse Dung
We’ve had quite a few sightings of bot larvae this week in horse’s droppings. Late Spring and early Summer is just the time of year for them to emerge as the little reddish armadillo type larvae that have overwintered in the horses’s stomach are ready for the next stage of their lifecycle.
They’ve been hunkered down for 10-12 months, feeding off the contents of the horse’s gut and maturing. At this time of year they detach themselves and are passed out in the horses’s dung.
Bots are usually only harmful to horses in large numbers. If you’re seeing them now they are naturally leaving of their own accord so there’s no action needed. Once in the open air they will pupate in the soil for 3-5 weeks before the adult bot fly emerges. The flies themselves only live for about 2 weeks; long enough to lay their eggs on the horse’s coat. Look out for these and be ready then with a bot knife to hand to scrape as many off as you can find before they get ingested into the horse’s mouth to continue their lifecycle.
Bots are best treated with a moxidectin or ivermectin wormer in the late autumn or winter after a hard frost which will kill off any last flies. Many people combine this with their winter wormer for encysted redworm.