Who’s eating codling moth?
Calculating predators’ per-capita consumption.
This article is part of a series on the multistate project “Enhancing Biological Control in Western Orchards.”
How big a role can predators play in controlling codling moth in fruit orchards? That’s a question that Dr. Thomas Unruh, geneticist with the U.S. Department of Agriculture in Yakima, Washington, wants to answer.
He hopes to identify which generalist predators are potentially the most useful natural enemies, based on their per-capita consumption of codling moths. His work is part of the multistate project “Enhancing Biological Control in Western Orchards,” led by Dr. Vince Jones, entomologist at Washington State University.
Monitoring consumption of codling moth larvae by predators is more problematic than assessing parasitism of a pest, because predators eat the evidence. So, Unruh’s strategy has been to catch them in the act of eating them, which is not easy since there are few codling moths to be found, even in an orchard with significant fruit damage.
Most of the current knowledge of codling moth predators comes from scientists in the 1920s and 1930s who would patiently sit in orchards for hours on end watching what happened. Unruh decided to put new technology to work. He tried using video to capture predators that ate cocooned codling moth larvae placed on pieces of wood on the orchard floor. There were problems with this, however. To conserve camera memory, recording was triggered by motion detection, but blowing leaves and shadows would start the filming. During more than 1,000 hours of recording, only 11 codling moths were attacked by predators—mainly by birds and mice. The camera caught one incident of three ants pulling together on one codling moth larva, and one earwig attacking a larva.
Predation by birds and mice is consistent with what those patient scientists observed long ago, Unruh said, as well as more recent experience in organic orchards where tree trunks have been banded to capture cocooned larvae. Often, the bands were torn apart by pecking or chewing.
Unruh said the lack of insect predation in his trials might be due to the artificial arena he had set up to observe them or to the time of year (June and July). Codling moth is a cryptic insect, and the places where they put the cocooned larvae were not very realistic. Or, it might reflect the normal situation in orchards.
In studies that Unruh conducted in the laboratory, Opiliones (daddy longlegs or harvestmen) and earwigs ate free-living codling moth larvae, but not cocooned larvae. It took anywhere from 4 to 24 hours for them to attack and eat a larva. It was a tough meal to catch for the daddy longlegs because the codling moth larvae fought back. In one case, the larva broke the daddy longlegs’s mouthpart and killed it. Unruh said codling moth larvae have mouthparts strong enough to burrow through Sheetrock. They frequently carve out grooves in the bark of the trees to form cocooning sites.
Carabid beetles (Pterostichis melanaria) and wolf spiders attack both free-living and cocooned larvae and proved voracious predators. They attacked and consumed the codling moth larvae within an hour of being presented with them but took up to 24 hours to consume the cocooned larvae.
But nature is different from what happens in the confines of a petri dish, Unruh said. Out in the orchard, the predator has to find the codling moth and must prefer it over some other kind of food. The video caught ground (carabid) beetles simply walking over a cocooned codling moth larvae many times without any apparent appetite for it.
Because of the technical difficulties with the video monitoring and the amount of time it was taking, Unruh decided not to continue. This season, he will put tethered wiggling codling moth larvae and cocooned larvae in an orchard, and check hourly to see if they’ve been attacked. Unruh believes that the cocooned larvae are not as susceptible to predation as the moving ones, but predators have much longer to target them. After a mature larva emerges from the protection of the apple, it usually finds a cocooning site within 24 hours and will spend the next six months in the cocoon. So, although cocooned larvae are less likely to be preyed on in any given day, there are many more days when they can be attacked.
Since it’s difficult to catch predators in the act, Unruh has also been working on ways to examine their gut content to find out if they have recently eaten any codling moth larvae. Ground-dwelling predators are caught in pitfall traps containing propylene glycol, which kills them and preserves their DNA. The pitfall trap consists of a drinking cup sunk into a hole in the ground, with the rim about a quarter to a half inch below ground level. A coffee can lid on nail spikes makes an elevated cover to keep irrigation water out, and also provides shade and seclusion that lure predators in.
Captured predators are ground up, and an analysis is done using a PCR (polymerase chain reaction) protocol that can detect a specific codling moth gene.
In experiments last year, 10 percent of the wolf spiders tested showed positive for having eaten codling moth within the previous 24 hours. (After 24 hours, the codling moth DNA breaks down and is not detectable.) The percentages for daddy longlegs, earwigs, and ground beetles were 6, 2, and 4 percent, respectively.
Unruh said the propylene glycol has proven to be marginal for preserving DNA inside their stomachs, and this year he will test other preservation fluids.
Ultimately, he hopes that if it appears that ground-dwelling insects can help control codling moth, growers might be able to change their practices to protect them from insecticides—for example, by choosing more selective pesticides, applying them at lower rates, or using sprayers that better target the trees, allowing less spray to reach the orchard floor. Understanding which are the most important predators will show where monitoring efforts should be focused.
He acknowledged that it’s also possible he’ll find that the amount of predation is trivial.
Studying codling moth is challenging, he said, recalling that in the past, he has gone as far as to cut down 16 trees in order to count the codling moths on them.
For more information, go to http://enhancedbc.tfrec.wsu.edu.