Submitted to: Environmental Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/14/1997
Publication Date: N/A
Citation: Interpretive Summary: Non-toxic pesticides that are safe for the environment are under commercial development as replacements for chemical pesticides. Biological pesticides based on bacteria have been sold for many years but still suffer from a lack of acceptance by the end user. One reason for the low acceptance is that biological pesticides are active for only a short time after application. Sunlight and rainfall inactivate the pesticides quickly and cause a loss of insecticidal efficacy. This manuscript reports on the use and comparison of four formulations of Bacillus thuringiensis or Bt, a bacterial pesticide, under modified field conditions. Tests demonstrated that several surplus farm commodities protected Bt from sunlight and from rainfall. Wheat gluten, corn flour, and casein, when added in small amounts to a spray tank containing Bt, formed films on the leaf surface and entrapped the Bt. These films resisted the effects from the environment, and Bt activity lasted more than twice as long as a commercial formulation of Bt applied at the same time. These formulations, if adopted by industry, should lead to a longer residual time for Bt and, thus, better acceptance by the end user.
Technical Abstract: The effects of natural sunlight and simulated rain on the residual insecticidal activity of Bacillus thuringiensis subsp. kurstaki formulations applied to field grown cabbage were measured. Simulated rain applied by a sprinkler irrigation system (3.2 cm) washed B. thuringiensis from the plants, causing an average 20% reduction in insecticidal activity across all treatments. Natural sunlight was manipulated by applying degrees of shade treatments. Black plastic provided protection from sunlight degradation for seven days while both clear plastic and no cover treatments lost insecticidal activity beginning two days after application of B. thuringiensis. Results from plots covered with clear plastic were similar to results from plots with no cover, indicating that clear plastic did not significantly change the effect of natural sunlight. There was no interaction between simulated rain and shade treatments and thus no synergistic loss of insecticidal activity by the combination of both environmental factors. Formulations consisting of 1% w/v gluten and 0.5% w/v casein resisted wash-off better than flour/sucrose (2% w/v) and Dipel 2X. Resistance to sunlight degradation was greatest with the gluten formulation and progressively less for casein, flour/sucrose and Dipel 2X formulations. Half-life of insecticidal activity in response to sunlight was calculated to be 7.1, 5.7, 4.8, and 4.3 days for gluten, casein, flour/sucrose and Dipel 2X formulations, respectively.