Some of our horses have had terrible insect problems on the midline of belly all summer. I do the cheap, Dollar Store Vasaline on them about every other day, to keep the crust gone and skin soft. Some have rough patches behind the girth area, another will have the whole stomach line crusted.
I have been getting the creamy Vasaline, has added Vit E, because it comes in larger tubes than the boxed, plain Vasaline stuff, found in the Baby section. For $1 the tube, I can afford to get LOTS of the Vasaline and grease the horse stomachs as needed. The creamy stuff seems to stay in place, not get runny, like plain Vasaline.
They also got some terrible bites up between the hind legs, both mare and geldings, that I just slathered the Vasaline on and they cleared up. Chests, armpits, between jawbones, also were bitten off and on as the insects changed during summer, so those parts also got greased as I found bites or scabs. Oddly enough the two younger horse have had almost no bites! We are blaming their color, not as dark of Bay as the older horses.
Ours are stalled in daylight hours, which helps reduce bites too. Fly spray of any kind doesn't really seem to help much, even the PRICY Endure spray. Have to soften the crusts and cover skin to protect the locations with the greasy layers of Vasaline to get ANY kind of improvement.
