Microbial pesticides, which are a class of biopesticides, consist of microorganisms such as bacteria, fungi, viruses and protozoans. The most widely used microbial pesticides are those derived from Bacillus thuringiensis (Bt), which is a naturally occurring soil bacterium. Individual strains of the bacterium produce a different mix of proteins that specifically kill one or a few related species of insect larvae. The most commonly used microbial pesticide is Bacillus thuringienesis subsp. kurstaki (Btk), which is only active on caterpillars that feed on greenhouse-grown vegetable crops (Figs. 1 and 2). A common product used in greenhouse vegetable production systems is Dipel (Fig. 3).
Bacteria, unlike fungi, must be ingested to be active. Therefore, thorough coverage of all plant parts, especially the upper and lower side of leaves on which caterpillars feed, is important so that residues of the bacterial spores can be ingested during feeding. Once consumed, the bacteria produce an endotoxin or crystalline protein that dissolves in the alkaline content (>7 pH) of the caterpillar stomach. The crystals then bind to the lining of the gut membrane, dissolve the stomach lining and create pores that result in leakage and swelling. The swelling continues until cells burst, which allows the highly alkaline gut contents to enter into the insect’s body cavity and bloodstream (hemolymph). This disrupts the blood, resulting in paralysis and death within 24 to 72 hours.
Btk must be applied when caterpillars are present, and young and small. When caterpillars are small, they do not have to consume as much plant material for the bacteria to be effective. Caterpillars must also be killed before they cause substantial plant damage and prior to reaching the reproductive phase (adulthood) of development. If Btk is applied later in the growing season when caterpillars are larger, it is too late because the caterpillars have to ingest much more plant material containing the bacteria to be effective. Furthermore, caterpillars may transition from the growing phase to the reproductive phase before consuming enough of the bacteria, which results in a higher probability of adult survival, leading to new generations of adults that will be produced in the future.
Btk has very short residual activity due to breakdown when exposed to sunlight (ultraviolet degradation). In addition, the wavelength of light transmitted may influence residual activity. For example, wavelengths greater than or equal to 400 nm can significantly reduce the viability (ability to survive and live) of Btk spores. Therefore, frequent applications and applications made in the evening are warranted in order to maintain residues of Btk spores on leaf surfaces. In addition, Btk is slower-acting than most conventional insecticides and must be applied before caterpillar populations reach damaging levels. Due to the selectivity of Btk, there is minimal potential direct harm to natural enemies including parasitoids and predators. However, Btk will kill host caterpillars, which can indirectly affect natural enemy populations.
The mode of action of Btk is specific to caterpillars, so there are no harmful effects to mammals. Btk must be stored under temperatures between 50 and 60° F to prolong shelf-life. Exposure to cold (<32° F) and hot (>90° F) temperatures will reduce the viability of the bacteria. Water quality may also negatively influence the viability of the bacteria. For instance, alkaline water, with a pH >8, will reduce the effectiveness of the toxin associated with Btk. The appropriate water pH should be =7.
It is important to not solely rely on Btk to suppress caterpillar populations because, like most insect and mite pests, using the same mode of action continuously can result in resistance. For instance, both the cabbage looper, Trichoplusia ni, and the diamondback moth, Plutella xylostella (Fig. 4) can develop resistance to Btk. Therefore, be sure to rotate Btk with other insecticides registered for use against caterpillars to mitigate any issues associated with resistance. In greenhouse vegetable production systems, applying Btk in conjunction with early releases of certain species of the parasitoid Trichogramma ssp. may be an effective strategy of maintaining specific caterpillar pest populations below damaging levels.