Bacillus thuringiensis (Bt) is a naturally occurring bacterium used widely as a biological control agent against specific insect pests, particularly those in the lepidopteran order, which includes moths and butterflies. The subspecies kurstaki (Bt-k) is effective against the larval stage of these pests, including the highly destructive Squash Vine Borer (SVB). When the caterpillar consumes the treated plant tissue, the alkaline environment in its gut activates the bacterial proteins, causing the larva to stop feeding and eventually die. Injection is a highly targeted application method that delivers the bacteria directly to the internal feeding site, making it one of the most effective ways to combat the SVB.
Identifying the Target Pest and Optimal Timing
The injection method is necessary because the SVB larva feeds entirely inside the squash stem, making it inaccessible to typical surface sprays. Scouting for the adult pest, a reddish-orange and black clear-winged moth, is the first step in prevention. The female moth lays tiny, brown, flat eggs near the base of the squash plant, often on the main stem or leaf stalks.
The optimal window for injection is immediately after the eggs hatch and the young larvae begin to burrow. This timing frequently coincides with the period when the squash plant first begins to flower and the vines start to run. Look for signs of infestation such as small piles of yellowish-green sawdust-like material, known as frass, expelled from the borer’s entry hole near the base of the stem. Treating the stem preventatively, or immediately upon seeing the first frass, greatly improves the chance of saving the plant.
Calculating the Recommended Dosage and Dilution
Determining the correct concentration involves scaling down the manufacturer’s directions for general spraying to create a much more potent, localized solution. Standard commercial liquid Bt products are typically mixed at a rate of approximately 1 to 2 teaspoons of concentrate per gallon of water for foliar application. For injection, this mixture is too dilute to be effective in the small volume applied to the stem interior.
The goal is to increase the concentration of the bacterial spores significantly to ensure the larva ingests a lethal dose within the limited space of the stem cavity. A common and effective ratio for injection is to mix the concentrate with water at a 20:1 ratio (20 parts water to 1 part Bt concentrate). For practical purposes, a small batch can be made by mixing approximately 1/2 teaspoon of Bt concentrate with 3 to 4 teaspoons of water. This highly potent solution is then drawn into the syringe for application.
The Step-by-Step Injection Procedure
The physical application requires a few specialized tools to ensure the solution reaches the internal target effectively. Acquire a disposable hypodermic syringe, commonly a 3cc or 5cc size, fitted with an 18 to 22 gauge needle. The larger gauge needle is preferred as it is less likely to become clogged by the liquid concentrate.
Locate the injection site either preventatively—about 1.5 to 2 inches above the soil line on the main stem—or directly into the stem just above the visible frass. Before injecting, clean the needle with an alcohol wipe to prevent introducing pathogens into the plant tissue. Insert the needle gently into the center of the stem, aiming to penetrate the vascular tissue where the borer is likely feeding.
Inject the solution slowly, administering a measured volume of approximately 1 cubic centimeter (1cc or 1 milliliter) per injection site. Injecting slowly prevents the solution from leaking back out of the injection hole, which wastes the product. If treating a long borer tunnel, multiple injection sites may be necessary, spaced a few inches apart along the affected area of the vine.
Follow-Up Applications and Stem Care
Since the Bt bacteria are only effective for a short time after mixing and must be ingested by the pest, follow-up applications are necessary. Re-inject the main stem weekly or every 7 to 10 days during the peak of the SVB egg-laying season to maintain protection against newly hatched larvae. It is important to mix a fresh batch of the concentrated solution for each application, as the potency of the diluted product degrades quickly.
After the injection is complete, focus on helping the plant heal and preventing secondary infections. You can seal the small injection hole with a small piece of electrical tape or a dab of pruning paint to protect the wound from fungal or bacterial pathogens. This sealing promotes faster healing and reduces the chance of the plant wilting due to the physical damage.