Why Predictive Models
Insects are cold-blooded animals and thus how fast they develop from egg to adult is driven by the temperatures they are exposed to. Degree-day models, that accurately predict the development of insects, have been used for several insects, including the codling moth. Since the early 1980s the codling moth degree-day model has been used to more precisely time insecticide applications.
In the past, a biological event, the first capture of codling moth adults in a pheromone trap, has been used to initiate the accumulation of degree-days. This biological event was referred to as the “biofix,” because it represented a biological fix point to initiated the codling moth model. Establishing a biofix for codling moth was a challenge in many orchards, therefore, WSU scientists developed a no-biofix model that accurately predicts the development of codling moth by accumulating degree-days from January 1 of each year. On average, the first emergence of codling moth adults starts at 175 (°F) degree days from January 1, so there is no need to use moth capture in pheromone traps to initiate the accumulation of degree-days. In the past, the degree-day total was set to 0 when the biofix, first moth capture, was determined. However, with the no-biofix model degree-days there is no reset of degree-days to zero.
Timing of pesticide treatments depends on the life stage targeted.
The Use of Degree Day Models
The Decision Aid System (DAS) incorporates this information into the models so that producers and consultants can easily see development and timing of applications.
225 degree timing
The first target is the codling moth eggs. Eggs begin to be deposited between 225-275 degree-days. It is possible to apply insecticides, referred to as residual ovicides, between the 225-275 degree-day period. These insecticides kill codling moth eggs that were deposited on top of residues. Some insecticides applied at this time will also kill leafroller larvae that are present in the orchard.
Delayed first cover timing
It is also possible to apply horticultural oil at 375 degree-days, a treatment that kills codling moth eggs already deposited. Horticultural mineral oil is referred to as a topical ovicide. If a residual ovicide or oil is applied as outlined above, then the next time to apply an insecticide would be at 525 degree-days. This timing if often referred to as a delayed first cover spray, which targets codling moth larvae hatching from eggs.
If no residual or topical ovicides are applied in the 225-275 or 375 degree-day timings, respectively, then insecticides should be applied at 425 degree-days, which targets the beginning of codling moth egg hatch period.
Repeat applications of insecticides in the first codling moth generation should be based on the need to suppress the pest population. The interval between successive insecticide treatments should be determined by the length of the active residue of the insecticide used. Timing for the second codling moth generation should also be based on degree-day accumulations. Moths of the second generation will being emerging at 1175 degree-days and first egg hatch will begin at 1400 degree-days. Control timing strategies used in the first generation can be applied in the second generation. The best way to utilize the codling moth model is to access the WSU Decision Aid System (www.decisionaid.systems). This computer based system automatically updates the codling moth model but it also provides management guidelines and is linked to the pesticide recommendations found in the WSU Crop Protection Guide for Tree Fruit – EB0419.
by Jay Brunner, originally published 1993, revised 2018