The Black Margined Blister Beetle, Epicauta pestifera, is one of a number of Epicauta species that contaminate hay and poison horses. Blister beetles are known for production of a toxin known as Cantharidin. Cantharadin is toxic to a variety of birds, and mammals.
Horses are especially sensitive to cantharadin. Feeding on hay contaminated with blister beetles can kill horses. In areas with high populations of blister beetles, farmers must be careful how they bale hay and make sure that contaminated hay is not fed to horses. Cutting the hay and allowing it to thoroughly dry before baling will encourage the blister beetles to leave and not be baled and crushed. Several species of Epicauta are implicated in these poisonings.
Some species of blister beetle are known as “Spanish fly” and sold as an aphrodisiac. Their supposed aphrodisiac properties stem from the irritation of mucous membranes. However, the aphrodisiac properties of cantharadin are over rated and Cantharadin is a blistering agent that is toxic in large quantities. The toxicity of cantharadin is a problem.
Cantharadin is toxic to many insects and repels predators. In many species of blister beetle, the cantharadin is produced only by the male and transferred to the female during mating. The female uses this “nuptial gift” to coat the eggs and protect the eggs from predators such as ants.
The Black Margined Blister Beetle lays eggs masses in the soil. The pale larvae (grubs) seek and feed on clutches of grasshopper eggs that the female grasshoppers lay in the soil. A large population of grasshoppers is often followed by increased populations of blister beetles. The blister beetle larvae will complete development on grasshopper eggs. The larvae pupate in the soil and emerge as adult beetles in spring. The adults can be active until late September and feed on foliage, flowers and pollen. The Black Margined Blister Beetle can be an occasional pest of potatoes by chewing on the foliage. The adults can aggregate and concentrate damage. These beetles are found in much of the eastern United States.
Black Margined Blister Beetle Feeding On Pollen
Living With Insects Blog 
This article mentions that the female of many species of blistering beetles get the toxin cantharadin, which protects her eggs from ants, from the male. This would mean that this would lend itself for competition amongst males. The female would try to find the male with the most of the toxin so that her egg sack is best protected. This would mean that they mate similarly to the dance fly.
Evolution is not goal driven, it is driven by random chance. If a female had a series of random mutations that led to a gene that made it prefer males with more toxin, your hypothesis could prove correct, assuming more toxin equals increased fitness. It will not occur simply due to the fact that it could provide an advantage.
It is really interesting that the male gives the female a toxin to protect the eggs. It is not very common in nature that the male participates in rearing or care of the young after fertilization in many different animal species. This is the male’s way of participating in care and ensuring that his efforts are as successful as they can be. It’s such a complex thing for the male to do, particularly since he is a small insect, and this would require a lot of effort for his body to produce this chemical when he might use the energy to mate more females. It must be a big advantage to him.
Is there an insecticide that could be used to target the genus Epicauta specifically and reduce toxic poisoning of horses?
There are a number of insecticides that would kill the beetles, but they are expensive. Farmers avoid insecticide treatments if there is a less expensive way to control pests.
I was very interested to read that the male gives the female the Cantharadin as a nuptial gift. It goes to show the marvels of evolution and instinct. The female would go on to use the toxin to ward off predators from her eggs and larva. it also shows the interdependency between genders of the blister beetles. I was also shocked to hear just how deadly the toxin can be to birds and mammals such as equine. A small dose of toxin that can flatten a large horse is something to be careful of. It was a bit humorous to read that these deadly bugs are also sold as aphrodisiacs. It’s ironic that these silent killers are used to stimulate intimate desires.
It is a very good idea for the female to cover her eggs in the Cantharadin. This will keep the predators from attacking the eggs. The blister beetles use their toxin in their defense and sometimes they do not use it on purpose. The article stated that the toxin can kill a variety of birds and mammals, such as horses. The mammals and birds are of no threat to the blister beetle, but they just happen to be eating what the blister beetles contaminate.