Left: A female Neoscona oaxacensis orb-weaving spider. Top: Cheiracanthium spiders are known as yellow sac spiders. They are usually pale colored and are 1/5 to 3/8 inch long. Gut-content analysis has shown evidence of feeding on codling moth. In central Washington, C. mildei is the species most commonly found. Bottom: A pair of Neoscona oaxacensis spiders. The smaller spider on the right, which is the male, appears to be courting the female in preparation for mating. He has his pedipalps extended toward her and these will be used to transfer sperm if she is receptive. Center: Tegenaria, or hobo spiders, pictured left, are fast-running spiders that make funnel webs. Gut-content analysis shows that they will feed on codling moth. Three species are found in central Washington, the most common of which is T. agrestis. Right: Spiders have spun webs all along this trellised orchard row in the conventionally managed Auvil Fruit Company orchard at Vantage, Washington.
The abundance of spiders in orchards has increased tremendously since growers have been shifting away from using the older, broad-spectrum pesticides, says Dr. Tom Unruh, entomologist with the U.S. Department of Agriculture in Yakima, Washington.
Large spider populations are particularly evident in organic orchards but are also being noticed in conventional orchards where more selective insecticides are used. Spider populations can be so high that people put pipes on the front of their four-wheel vehicles to keep webs out of their faces as they drive through the orchards.
The spiders have more than a curiosity value, Unruh believes. Spiders are generalist predators that will eat anything that comes their way if it’s an appropriate size— meaning it’s smaller than them—says entomologist Dr. Gene Miliczky, a colleague of Unruh’s. Young spiders have a different menu than older ones. Small spiders might feed on thrips, mites, and small aphids. Bigger ones might eat grasshoppers, wasps, bees, butterflies, or moths, including codling moth larvae or pupae.
In fact, they’re not at all picky about their food and will also eat lacewing larvae, lacewing nymphs, and other spiders. But Miliczky said that since pests tend to be more abundant in the orchard than beneficial insects, pests probably make up most of their diet.
Unruh and Miliczky are working on a research project to find out how much spiders might be contributing to codling moth control. Unlike parasites that might consume the innards of their host and leave some evidence, spiders eat their prey, making it difficult to tell what they’re attacking. So, the scientists are examining the gut contents of spiders to find out if they’ve recently eaten codling moth larvae or pupae.
Miliczky’s role in the project is to capture spiders in orchards and identify the species, while Unruh is in charge of analyzing their gut contents.
Miliczky has been capturing and identifying spiders since he joined the Agricultural Research Service in Yakima in the mid-1990s. He estimates he has found 35 to 40 different spider species in apple and pear trees and that many or more species on the ground.
Some spiders, such as wolf spiders, live on the ground and are rarely seen in canopy. These are captured with sweep nets or pitfall traps, which consist of plastic cups sunk into the ground. Spiders that live in the trees are caught on beating trays or by searching for webs in the trees. The species of spiders vary depending on the location and the insecticides used.
When he began collecting spiders, Miliczky might find a total of one or two spiders from 25 to 30 beating tray samples in a typical conventional orchard, compared with up to 100 from the same number of beating tray samples in an organic orchard.
Use of insecticides is the number-one factor that determines the abundance of spiders in an orchard, he said. “I think probably regardless of what you do otherwise, if you’re still using insecticides heavily, you’re going to harm your spider populations and other beneficials as well.”
Joe Shelton, orchard manager at Broetje Orchards in Prescott, Washington, said his company has both conventional and organic orchards. He first noticed spiders in the organic blocks, but he now sees them in the conventional blocks also. In the organic, the trees are full of webs.
“You walk through the orchard, and the webs hit you in the face,” he said. “It’s a big difference from how it used to be.”
Spiders develop through one generation per year and overwinter in either an immature stage or as eggs. Miliczky said, typically, the highest populations are found in August and September although some species are abundant earlier in the summer.
Live spiders that Miliczky collects for gut-content analysis are put in an ice chest, and kept frozen at the research lab until they can be analyzed, so they don’t continue to digest what they’ve eaten. The spiders are ground up individually, and the homogenates are analyzed in a PCR (polymerase chain reaction) machine that amplifies codling moth DNA. Sixteen different species of spiders have been analyzed, of which seven species had eaten codling moth.
Overall, 10 percent of the spiders analyzed so far had codling moth in their guts, but the percentage varies greatly by species. For example, 50 percent of the Holoena nedra had eaten codling moth. Holoena nedra is a fairly large funnel web spider that spins its webs on large tree limbs. It sits at the back of the web ready to pounce on anything that is caught. Other species that had fed on codling moth are: Phidippus, Antradiatus, Trochosa, Cheiracanthium, Paenius, and Teganaria.
Unruh said the study suggests that more selective pest management programs, incorporating granulosis virus, mating disruption, or attract-and-kill strategies, could increase the role of spiders and other beneficial insects in controlling pests.
Although laboratory assays have shown that some of the newer pesticides, particularly Delegate (spinetoram), can be toxic to spiders, they tend to be less disruptive than the older pesticides, Unruh said. Altacor (rynaxypyr) had no negative effect on the spiders tested.
“We have all these things that can potentially assist in the control of codling moth, and the more sustainable our management system, the more likely that’s going to be the case,” he said.
Also working on the project are lab technicians Kelly Archer, Linda Ray, and Aaron Etherington. Funding for the project has been received from the Washington Tree Fruit Research Commission and from a federal Specialty Crop Research Initiative grant.