Wildly Fluctuating Temperatures Affect Insect Flight Patterns

(revised from "Oriental fruit moth: next generation or tail end of the first one?", June 8, 2006, by Hannah Fraser, Neil Carter, and Kathryn Carter)

It seems that the fluctuating spring weather we've experienced this year has resulted in some difficulties in interpreting insect captures in pheromone traps and some confusion on where we are in terms of the generations of a few moth pests. One specific example we'd like to discuss this week is oriental fruit moth (OFM), but many of the concepts can also be applied to other moth pests.

Heat drives the growth and development of insects and mites. Insect growth and development can be inhibited at both minimum and maximum temperatures. Knowledge about the base temperatures required to drive development in specific insects has been used for decades to help optimize the application of pest control products. In addition to its influence on insect growth and development, temperature can have a profound effect on insect activity.

Just as there is a base temperature below which no development occurs in immature forms, there is also a minimum temperature required for activities including adult flight and searching for a mate. When it's too cold, adult OFM are not active in the orchard. They are not looking for mates; females are not laying eggs. But they are still out there. When temperatures rise, mated females get back to laying eggs. In addition, some of the overwintering generation that didn't quite make it to adulthood prior to things cooling down will continue their development and emerge to mate and lay eggs. These "late bloomers" represent the stragglers (sometimes in high numbers) that didn't make it to adulthood before the cool temperatures hit.

The result can be what is being reported in some tree fruit production areas this spring. It was warm, OFM flight was on the upswing and an apparent peak had, in some cases, been reached. Then things got quite cool, enough so for moth activity to cease. But it's been hot - really hot - over the last week. With several days of hot weather, adults are once again being caught in pheromone traps.

Many people have wondered if the "new" activity marks the beginning of the second generation of OFM. The answer is "no". There haven't been enough heat units (degree days) accumulated for this to have occurred. OFM have a base development temperature of 7.2ºC (45ºF) and require an accumulation of about 550 DDC base 7.2ºC (963 ± 46 DDF base 45ºF) to go from an egg to an adult (generation time). As of last Friday, when the question came up), we hadn't reached that yet. So … the adults that are being caught now represent the tail end of the overwintering generation. The same thing happened in the spring of 2006 - we know, because we wrote a very similar article at this time two years ago and we've been able to borrow great deal from it!

It's during years such as 2008 that we need to stress the importance of using as many monitoring tools as possible. Pheromone traps will provide information on insect activity, but in order to understand what's actually happening at a population level will also require the use of degree-day models. Use available degree day models to help time insecticide sprays, but keep checking those pheromone traps (don't take them down too early) to track extended or unusual activity! While it can be tempting save time and money by not putting traps up for overwintering generations of OFM (since timing for sprays coincide with petal fall in apple), there can be some risks associatied with this. If traps aren't up in the orchard to note delayed emergence important spray applications may be missed.

All of this can lead to challenges in management, and in some cases, additional insecticide sprays by the grower. If numbers being caught in traps haven't tapered off yet and the last spray was a couple of weeks ago, a second spray may be required to protect shoots and / or developing fruitlets. All growers should be aware of the residual activity for the pesticides they are using. Keep in mind that at this time of year, the trees will outgrow the residues quickly. If the pesticide has worn off, and traps are still catching a significant number of OFM (for arguments sake, let's say more than a third to a half as many as during the first peak), then a second application of insecticide should be applied. For resistance management, make sure that the 2nd insecticide application is the same product that was used for the first spray - don't rotate chemistries within a generation (rotate between generations instead).

Be aware that a split or bimodal peak in the overwintering generation usually translates to the same thing occurring in the second flight. For this reason, in areas where a bimodal peak (also called a "peak and shoulder") has been observed, growers using insecticides to manage OFM should be prepared to apply two sprays to manage the second generation, as indicated by activity in pheromone traps during the next flight period. Growers who are using mating disruption for the management of OFM in apples and / or tender fruit have the luxury of not having to worry about this.

Learn more:

• Oriental fruit moth: next generation or tail end of the first one?