Leafrollers have a number of parasitoids that naturally occur in orchards. Many of them are parasitic wasps. However, Tachinid flies cause a significant amount of parasitism in leafrollers. Tachinid flies look very similar to houseflies with long, bristly hairs on their abdomen.
Parasitoids are insects whose larvae live as parasites inside other insects, eventually killing their host. Adults are typically free-living while the parasitic immature stages grow and feed inside their host and tend to kill their host just as they complete their own development. Parasitoids are often highly specialized and attack only one or a few types of insect pests, making them important biological control agents.
But how relevant are these parasitoids for controlling leafrollers in the orchard? Studies in WA orchards have shown that 19% of the overwintering obliquebanded leafroller population and 29% of the summer generation are parasitized. For the Pandemis leafroller, the mortality due to these parasitoids is even higher, with 29% and 45 % in the overwintering and summer generations, respectively.
One might say, “Very well, but the parasitized leafroller larvae still actively feed in the orchard. So, what’s the benefit, if they don’t kill the leafrollers right away?” Looking at the bigger picture, the parasitoids significantly reduce the number of leafrollers that survive into adulthood and, therefore, the number of eggs laid for the next generation. Leafrollers can be tolerated in low densities as they mostly feed on foliage, and only feed on fruit when it touches leafroller-infested leaves during the summer generations. Overall, reductions in one generation carry over to the following generations, reducing the pest pressure over time.
When given the chance, leafroller parasitoids can help suppress the leafroller population, consequently reducing the need for pesticide applications. Key to this strategy of relying on parasitoids for parts of leafroller management is to use very selective insecticides (not only for leafroller control) or no insecticides at all when the parasitoids are most active in the orchard.
Generally, the application of non-selective insecticides should be avoided when leafroller larvae are in the 4th through 6th instar and pupal stage. Not only is that the time when parasitoids are actively looking for hosts, larvicides applied during that time are less effective in killing the larger immature stages. If not spraying or delaying the spray is not an option, then selective materials, such as Bt against young leafroller larvae or granulosis virus against codling moth larvae, are favorable in order to protect the natural enemies.
Click on the image to enlarge it.
(Ute Chambers, WSU-TFREC)