Tractor speed needs to be low enough to allow the spray to thoroughly cover all foliage. Good control can be achieved with both dilute and concentrate applications. If coverage is good, concentrate applications may be the best choice because the Bt toxin concentration is higher and larvae are more likely to consume a lethal dose. Proper Timing at the Right Temperature Proper timing of Bt sprays will greatly improve their effectiveness. As a general rule, Bt's should be timed to coincide with periods when leafroller larvae are most likely to be feeding.
Field studies indicate that feeding activity increases during warm weather.
Dr. Alan Knight has recently found that Bt treatments in the spring were approximately 1.5 to 2.0 times more effective with each 10 F increase in the daily maximum temperature. A general recommendation is to apply Bt's when daytime high temperatures are expected to be at least 65 F for a day or two following the spray. Thus, Bt applications should be delayed until weather is warm, rather than included as part of a spray applied for other reasons during cool weather. For the major leafroller pest in Michigan, obliquebanded leafroller, this means that petalfall or later applications of Bt may be more effective than those applied earlier in the season. Don't Cut the Rate Using full label rates of a Bt product generally provides the best control.