The Squash Vine Borer (SVB) is a destructive pest that can swiftly kill squash plants by feeding internally, making it notoriously difficult to control. Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium widely used in biological control. Bt functions as a highly specific insecticide that must be consumed by the pest to be effective. This article explores the biological mechanism of Bt and determines its efficacy against the internal-feeding SVB.
Identifying the Squash Vine Borer and Its Life Cycle
The adult Squash Vine Borer is a day-flying moth often mistaken for a wasp due to its clear hind wings and distinctive orange and black markings on its abdomen. This clearwing moth emerges in early summer, typically around late June or early July, marking the beginning of the egg-laying period. Female moths lay small, reddish-brown, oval-shaped eggs, usually singly, on the stems and leaf stalks near the base of the squash plant.
The destructive stage begins when the eggs hatch, which occurs in about one to two weeks. The newly emerged larvae, which are creamy white grubs with dark heads, immediately bore into the plant’s stem within hours of hatching. Once inside, the larvae feed on the vascular tissue for four to six weeks, causing the plant to suddenly wilt and die due to the disruption of water and nutrient flow. The presence of sawdust-like frass, or excrement, near the base of the plant is the most common sign that the larva is already inside.
The Specific Action of Bacillus thuringiensis (Bt)
Bacillus thuringiensis is a spore-forming bacterium found naturally in soil and on plants, used globally as a biological insecticide. The specific strain used to target the Squash Vine Borer is typically Bt kurstaki (Bt-k), which is effective against caterpillars, a group that includes the SVB larva. Bt does not kill on contact but must be ingested by the susceptible insect to work.
When the SVB larva consumes plant tissue treated with Bt-k, it also ingests the bacteria’s microscopic protein crystals, known as Cry toxins. The highly alkaline environment of the caterpillar’s midgut is the specific trigger that dissolves these crystals and activates the toxin. The activated Cry toxins then bind to receptors on the gut lining cells, creating pores in the membrane.
This process paralyzes the digestive system, causing the larva to stop feeding almost immediately, and leads to the breakdown of the gut wall. The gut contents, including the Bt spores, leak into the insect’s body cavity, causing a lethal infection. Since the digestive systems of humans, mammals, birds, and beneficial insects are acidic, the Cry toxins do not activate, making Bt a highly selective and targeted control method.
Effective Application Techniques for Internal Borers
The internal feeding behavior of the Squash Vine Borer makes simple external spraying of Bt largely ineffective once the larvae are inside the stem. To successfully use Bt-k against this pest, the application must be targeted to reach the borer before or shortly after it enters the vine. The most effective strategy involves an injection method, often using a syringe without a needle, to deliver the solution directly into the stem base.
This prophylactic injection should be performed into the stem one to two inches above the soil line, or into the leaf petioles of the primary vines. The solution is drawn into the plant’s vascular system, making the internal tissue toxic to the feeding larva. Because Bt-k breaks down quickly in sunlight, losing effectiveness within days, the timing and frequency of application are paramount.
A regular injection schedule, repeated every seven to ten days, is necessary throughout the period of adult moth activity and egg-laying. This routine ensures that any newly hatched larva that bores into the plant encounters a fresh dose of the bacterium. If an infestation is already visible, the Bt solution can be injected directly into the bore hole or slightly above the frass to target the feeding larva.
Complementary and Non-Chemical Control Strategies
While Bt injection can be an effective tool, integrating non-chemical methods offers a more robust defense against the Squash Vine Borer. One of the most successful preventive measures is the use of floating row covers, which act as a physical barrier to exclude the egg-laying adult moths. These covers must be applied immediately after planting and removed once the plants begin to flower to allow for pollination.
Cultural practices also significantly reduce the risk of infestation, such as rotating the location of squash crops each year and practicing fall sanitation to destroy crop residues. Fall tillage, which involves turning the soil, can expose overwintering pupae to the cold or to predators, helping to suppress the next season’s population. Planting resistant varieties, particularly those in the Cucurbita moschata species like butternut squash, offers a natural defense since their denser vines are less preferred by the borers.
For plants already infested, a physical removal technique, often called “squash surgery,” can save the plant. This involves carefully slitting the stem lengthwise at the point of frass to locate and remove the larva. The wounded stem should then be buried with soil to encourage new root growth. Monitoring adult moth activity with pheromone traps is another valuable strategy, as catching the first male moths signals the time to begin preventative interventions like applying Bt or installing row covers.