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 typically have two distinct generations per year. 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.
In general, there are two windows for control of OBLR larvae, one in each generation. Control in the spring period is more variable because of variability in the weather and potential rainfall effects on residual longevity, but sprays at this time generally have better coverage, and populations at this time do not generally feed on the fruit with the frequency that happens during the summer. Control of either generation can give good reductions in OBLR populations and it is unlikely you would need to target both generations unless there is a significant migration from sources external to your orchard.
In both the spring and the summer, plant growth tends to reduce the residual longevity because the larvae move towards the tip of the shoots and feed there were no residue is present. Thus, the efficacy of either conventional or organic materials is often fairly similar and the more frequent application of the organic materials like Bt gives good control if multiple applications are applied. It is important that Bt be applied only when the high temperatures are 60°F or greater because larvae tend to feed less at lower temperatures and must ingest the bacteria to be effective.
Optimal OBLR Control
In the spring generation, you need residual activity to cover the period from 135 to 300 DD if your population is very high or fruit damage occurred the year before. This would generally be 3 Bt sprays (applied 7 days apart) in organic orchards or 1-2 conventional sprays because of plant growth. It is unlikely that you need to do this every season as leafroller populations vary considerably from year to year and if you target the first generation with sprays during this period, you should not have problems in the second generation unless you have a large amount of movement into your orchard from external sources. The WSU Decision Aid System provides the best timing for sampling to help determine if populations require treatment or not.
If only the second generation is targeted, the population is higher and getting good coverage is more difficult, but the residual activity is less likely to be affected by rainfall than in the spring. In addition, the degree-day accumulations per day are much higher than in the spring, which means that even though you need to cover a longer period on a degree-day basis, a residue of 7-14 days controls a larger part of the population. In the summer, the residual should cover the crop from 955 to 1300 DD if the population was very high or fruit damage occurred the year before. This would likely be 3 sprays of Bt applied a week apart in organic situations and one optimally timed conventional pesticide with a residue of 17-21 days for reasonable control. It is unlikely that you would need more than two conventional sprays or 3-4 organic (Bt) sprays all year to control OBLR in most situations unless your timing and/or coverage is poor or you have a large external population of OBLR moving into your orchard. Consult the WSU spray guide on DAS for best materials to use for OBLR control, or full Crop Protection Guide.
Pesticide effects on Biological Control
In the timing of insecticide applications for OBLR, try to avoid sprays that go on in the fourth instar or later as much as possible. This is because parasitoids of OBLR tend to attack the fourth and later instars. Sprays of Bt do not affect parasitoids other than by eliminating the OBLR larvae, but conventional sprays or Entrust sprays may affect the adult parasitoids. Check the Orchard Pesticide Effects on Natural Enemies Database (OPENED) or the full spray guide on DAS for the non-target effects of different materials used for OBLR control.
Prepared by Matthew Jones, Ute Chambers, and Vince Jones, May 2019