Timing is everything when it comes to effective pest management. Knowing when a pest is susceptible to which kind of pesticide is the key for successful control.
Leafrollers have two to three distinct generations per year, depending on the temperatures accumulated. This makes it easy to predict what developmental stages the population is going through at various times of the season. In turn, this determines what management tactics are effective and when.
Dr. Vince Jones, Prof. in Entomology at Washington State University, recently developed a population model for obliquebanded leafroller (OBLR). With input of mortality caused by different pesticides, the model can simulate when OBLR populations are most and least sensitive to pesticide applications throughout the season. Coupled with OBLR phenology, Dr. Jones identified four windows in time when OBLR control is effective:
- 90 degree-days (DD) – larvicide application, when overwintering larvae are becoming active in spring,
- 720 DD – ovicide application (e.g. oil), when eggs of the first summer generation are being deposited,
- 900 DD – larvicide + oil application, when eggs and young OBLR larvae are abundant, and
- 1800 DD – larvicide + oil application, when eggs and young OBLR larvae of the second summer generation are abundant.
Any other application timing would result in low or no control efficacy. (See the box for more detailed information on these four windows.)
The next step for Dr. Jones was to simulate the effects of different spray programs that applied pesticides at two, three, or all four of these control timings. Population mortality was used as a measure of spray program efficacy.
The simulations revealed that larvicide + oil applications at 900 DD are the most important timing for OBLR control and should always be part of any spray program. Applying pesticides at any three of the four windows (including the 900 DD timing) results in similar control efficacies, while hitting all four windows increases efficacy only marginally.
Furthermore, the simulations demonstrated that applications of Bt at 7-day intervals are as effective as the use of conventional larvicides. Shoot growth needs to be considered in determining re-application intervals of spray materials.
The perhaps most significant finding of Dr. Jones’ study is that OBLR treatments at any three of the four degree-day timings is considerably more effective than the old standard OBLR program, which used larvicide + oil applications at 370 DD, 1350 DD, and 1800 DD.
To make the shift in OBLR management strategy easier, the DAS management recommendations for OBLR now reflect the new and more effective control timings.
Optimal OBLR Control
The first window that can be protected occurs from 90-160 DD, and treatment should consist of a larvicide (either something like Entrust or Bt). As this is early in the year, only use Bt when the maximum temperatures are predicted to be above 65°F for 3 days after application. Also, if you use Bt, three sprays will be needed 7 days apart. Oil at this time does not improve efficacy.
The second window provides an opportunity to use an ovicide at 720 DD. This is the least effective window by itself, but increases control when combined with other treatment timings. If oil is uses as the ovicide, it only affects the eggs that are present at this point in time; ovicides such as Intrepid have an effect on both eggs present at time of application and those laid on the residue for at least 10 days. This treatment would be unlikely to be cost-effective in most situations.
The third window is the most important one. Treatments should include an oil + a larvicide. The window starts at 900 DD, and the spray activity needs to cover the period from 900 to ? 1450 DD. If using Bt, three treatments spaced 7 days apart are required, but a fourth treatment may be necessary if populations are high. If using Bt, only the first two sprays need to have oil added. If using a conventional pesticide or Entrust, an oil + a single spray of the normal leafroller larvicides are likely to be the only treatment necessary, unless the trees are undergoing rapid stem elongation and the population is totally out of control. If a second treatment is applied, no oil would be needed and only the new foliage growth needs to be covered well. Studies have shown that the activity of most larvicides at this time of year is extremely long (?50+ days), so older leaves are well protected.
The fourth and final treatment window occurs right before harvest at 1800 DD and requires a larvicide + oil treatment. This timing is likely to be useful only if the population is really high and leaves are in contact with the fruit. Treatments at this time will suppress the overwintering population, and potentially reduce damage from those instars that made it through the sensitive period for diapause induction and continued on to a “suicide” generation unable to overwinter. However, sprays covering the 900-1450 DD period should have controlled the population.
(Ute Chambers and Vincent P. Jones, WSU-TFREC)