Blister beetles, belonging to the insect family Meloidae, are plant-feeding insects easily identified by their narrow bodies and a distinct neck-like structure behind a broad head. These beetles possess a potent defensive compound called cantharidin. This odorless, colorless substance is highly toxic when ingested and causes severe blistering upon contact with skin, which is the source of the insect’s common name. Understanding the factors that draw these insects to a specific location is important for managing their presence and mitigating the risk associated with this toxin.
Feeding Preferences and Plant Hosts
The most direct cause of blister beetle presence on a property is the availability of their preferred food sources, primarily the pollen and nectar of flowering plants. Adult blister beetles display a strong attraction to vegetation that is in bloom, often congregating in large, concentrated clusters on host plants. This gregarious feeding habit makes them a significant agricultural pest, as their numbers can rapidly strip foliage or contaminate forage crops.
A major attractant in agricultural settings is alfalfa, especially when it reaches the bud or bloom stage of development. Plants in the legume family, including clover, soybeans, and sweet clover, are readily sought out by several common species. In gardens and vegetable patches, blister beetles show a preference for plants in the nightshade family, such as potatoes, tomatoes, and eggplant. Beyond cultivated crops, the beetles also feed on a wide variety of flowering weeds and ornamental plants. Specific high-risk plants include asters, goldenrod, pigweed, and puncturevine.
Habitat Requirements for Reproduction
Blister beetles are drawn to a property for reasons beyond just food, as certain environmental conditions are necessary to support their unique reproductive life cycle. The larvae of many common species are predators, and their presence is directly linked to the availability of a specific food source for this stage. Female beetles lay masses of eggs in the soil in late summer, and the resulting larvae, called triungulins, immediately begin searching for prey.
The primary food for the larvae of many pest species, such as those in the genus Epicauta, is the egg pods of grasshoppers. Consequently, properties or fields that have recently experienced high grasshopper populations often see an increase in blister beetles the following season. This connection means that areas like pastures, rangelands, and undisturbed field borders serve as reproductive attractants. The female beetles also require specific soil conditions for oviposition, favoring loose or sandy soil where grasshoppers typically deposit their egg masses. The larval stage provides a form of natural biological control against grasshoppers, but it ultimately leads to a localized emergence of large numbers of adult blister beetles the following summer.
Sensory Cues and Aggregation Signals
Once food and reproductive sites are established, blister beetles use specific sensory cues to locate one another and aggregate into their characteristic swarms. Like many insects, they are often phototactic, meaning they are attracted to light, which can draw them to porches, barn lights, or vehicle headlights at night. This attraction to exterior lighting is a common reason for their sudden appearance near homes and outbuildings. Visual cues also play a role, as certain species may be attracted to specific colors, such as sky blue, which reflects light over a broad range.
The sudden appearance of large groups is heavily influenced by chemical communication, or semiochemicals, which facilitate their gregarious behavior. While the exact volatile aggregation pheromone for all adult species is not fully characterized, the females of some species emit a sex pheromone to attract males for mating. Furthermore, the defensive compound cantharidin itself is known to be involved in their chemical communication.