Bacillus thuringiensis (Bt) is a naturally occurring bacterium used as a biological pesticide by gardeners and agriculturalists. This microbe is known for its selective toxicity, targeting specific insect groups while leaving beneficial organisms, wildlife, and humans unaffected. Since Bt is widely used against many common garden pests, people often question if it can manage grasshoppers (order Orthoptera). The answer depends on the highly specific biological mechanism of the Bt toxin, which relies on the unique digestive physiology of susceptible insects.
How Bacillus thuringiensis Works and Its Target Pests
The power of Bacillus thuringiensis comes from the protein crystals, known as Cry toxins, that the bacterium produces during its spore-forming stage. These crystals are initially inactive (protoxin form) and pose no threat until ingested by a susceptible insect. For the toxin to become lethal, it must be dissolved and activated within the insect’s digestive system.
This activation requires a highly alkaline environment, specifically a pH range of 9.0 to 10.5, found in the midgut of certain insect larvae. Once dissolved, specific digestive enzymes (proteases) cleave the protoxin, converting it into the smaller, biologically active toxin. The active toxin then travels to the midgut lining where it binds to specific receptor sites on the epithelial cells.
Binding to these receptors leads to the formation of pores in the gut cell membranes. This pore formation disrupts the digestive system, causing gut contents to leak into the body cavity, leading to paralysis, an inability to feed, and death within a few days. Because the mechanism is precise, different Bt strains target different insect orders. For example, the kurstaki strain (Bt-k) targets Lepidoptera larvae (caterpillars), while the israelensis strain (Bt-i) is effective against Diptera, such as mosquitoes and black flies.
Why Bt Toxins Do Not Affect Grasshoppers
The fundamental reason Bacillus thuringiensis products do not harm grasshoppers is that their digestive system lacks the two necessary biological conditions for toxin activation. Grasshoppers (order Orthoptera), including crickets and katydids, have a distinctly different gut environment than susceptible insects. The Bt protoxin requires a highly alkaline midgut pH of 9.0 or higher to solubilize, but the grasshopper gut lumen is only slightly acidic to neutral.
Studies show grasshopper gut pH ranges generally from 5.6 to 7.5 across the digestive tract. This neutral-to-slightly-acidic environment is insufficient to dissolve the Cry toxin crystals, meaning the protoxin remains inert and passes harmlessly through the insect. Furthermore, grasshoppers lack the specific molecular receptors on their midgut epithelial cells required for the toxin to bind and form pores.
The toxicity of Bt depends entirely on the presence of these particular receptors, which are unique to susceptible insect orders. Without the correct chemical environment for activation and the necessary binding sites, commercially available Bt strains are entirely ineffective against grasshoppers. Relying on Bt is not a viable strategy for controlling grasshopper populations.
Proven Methods for Grasshopper Management
Since Bacillus thuringiensis is not an option, grasshopper management requires alternative strategies proven effective against Orthoptera.
One biological control method uses the protozoan Nosema locustae, available commercially in a bran bait formulation (Nolo Bait). When grasshoppers consume the bait, the pathogen infects their fat bodies, causing death and reducing the population’s overall fertility and lifespan.
For smaller populations or garden protection, cultural controls can be employed. This includes careful tilling of breeding areas in late fall or early spring. Tilling disrupts the soil and exposes grasshopper egg pods to desiccation, weather, and predators, significantly reducing the number of nymphs that hatch. Protecting desirable plants can also be achieved using physical barriers like mesh row covers or aluminum window screening.
Chemical control, often used for heavy infestations, should be targeted at young grasshopper nymphs, typically in the third and fourth instar stages. Insecticides like carbaryl or pyrethroids are most effective when applied to the perimeter or border areas of the yard or garden, before the pests move into the protected areas. Treating the perimeter ensures that the young hoppers are intercepted in their hatching grounds, maximizing control before they mature into mobile adults.