We are receiving several reports of blister beetles in South Dakota. While the larvae of these insects are considered beneficial due feeding on grasshopper eggs, the adults can cause defoliation as well as health issues for livestock. Blister beetle populations will typically follow areas that had large grasshopper populations during the previous growing season. Defoliation caused by blister beetle adults is usually minor. However, the main concern with these insects is that the adults contain a chemical known as cantharidin that is capable of producing painful blisters. Cantharidin is present in blister beetle hemolymph (blood) but can be exuded through reflexive bleeding when pressure is applied to the beetle. This chemical can prove fatal when ingested in large quantities by livestock, especially horses, due to the irritation it causes in the gastrointestinal and urinary tracts of the animals.
Although there are several species of blister beetles found in South Dakota, they vary in their toxicity to livestock and their ability to defoliate crops (Table 1). The three-striped blister beetle contains a higher concentration of canthardin, while the margined blister beetle is noted for having the greatest potential to cause severe defoliation.
Table 1. Relative blister beetle toxicity to horses.
|Species||Estimated number of blister beetles required to cause mortality by horse weight|
|275 lbs||500 lbs||825 lbs|
|Black blister beetle||175||1,100||1,700|
|Three-striped blister beetle||40||80||120|
*Adapted from Capinera et al. 1985. J. Econ. Entomol. 78: 1052-1055.
As their name implies, striped blister beetles have black stripes that run the length of their yellow/orange abdomens. Striped blister beetles have an orange head with two oval markings that are separated by an orange line. The thorax, which is narrower than both the head and abdomen, is grey with two black lines on it. They have black antennae and gray/black legs. All blister beetles have elytra that are soft, and generally do not cover the end of their abdomen (Figure 2).
Figure 2. Striped blister beetle adult. Courtesy: Adam Varenhorst
If blister beetles are present in alfalfa prior to cutting, there are a few options for reducing the risk of livestock poisoning. The first option is to cut the hay without using a conditioner. Since the chemical is released when the beetles are crushed, hay conditioners can actually increase the amount of canthardin present in the hay. In addition, allowing the hay to dry completely after cutting gives adults an opportunity to crawl away prior to raking and baling.
Insecticides can also be applied to reduce blister beetle populations. However, when the insects are sprayed, they often clutch onto plants and may still be present in the hay at the time of harvest.