—by Kate Prengaman

Codling moth granulovirus does not kill larvae on contact, so growers who want to evaluate its efficacy need to cut open apples with obvious damage to see if the larva inside is dead, as in this photo. The black, desiccated larva is clearly dead, an indication that the virus sprays are working, according to entomologist Teah Smith, pest consultant for Zirkle Fruit. She uses photos like these to train scouts. (Courtesy Jose Lopez/Zirkle Fruit)
Codling moth granulovirus does not kill larvae on contact, so growers who want to evaluate its efficacy need to cut open apples with obvious damage to see if the larva inside is dead, as in this photo. The black, desiccated larva is clearly dead, an indication that the virus sprays are working, according to entomologist Teah Smith, pest consultant for Zirkle Fruit. She uses photos like these to train scouts. (Courtesy Jose Lopez/Zirkle Fruit)

Washington’s organic apple growers depend on codling moth granulovirus, often applying over a dozen sprays a year, to kill larvae and reduce populations and pest pressure. 

But recent research uncovered two populations of codling moth that have developed resistance to commercial formulations of the virus, said Anne Nielsen, an entomologist at Rutgers University who conducted the trials. 

To determine the extent of the resistance, Nielsen has partnered with Washington State University entomologist Tobin Northfield to collect additional samples from orchards reporting a lack of efficacy from their virus applications. 

Resistance to the virus, officially known as Cydia pomonella granulovirus, or CpGV in scientists’ shorthand, has been well documented in Europe for almost 20 years. Certis Biologicals, which manufactures granulovirus products under the trade names Cyd-X and Madex in the U.S., reached out to Nielsen about testing for resistance in 2018. She traveled to Germany to learn research techniques from the experts there, and in 2019 she identified a population with type II resistance. 

Recent failures to manage for the pest spurred her to examine additional populations, and she uncovered a second resistant population in a different part of Washington.

This apple hosts a living larva, distinguished by its light brown color and plump appearance. Finding larva like this in orchards sprayed with virus suggests a lack of efficacy. (Courtesy Teah Smith/Zirkle Fruit)
This apple hosts a living larva, distinguished by its light brown color and plump appearance. Finding larva like this in orchards sprayed with virus suggests a lack of efficacy. (Courtesy Teah Smith/Zirkle Fruit)

“Now that there are two, it’s not just a fluke,” Nielsen said at the Washington Tree Fruit Research Commission meeting in January. The commission is funding some of the research. 

There are several genotypes of the virus, isolated from different codling moth populations, known as groups A, B and E. The Cyd-X line of products uses group A. Virosoft products, manufactured by BioTEPP, use group E. Moths with type II resistance are resistant to both.

The good news: The two resistant populations identified so far are both susceptible to a new virus isolate that contains all three genotypes — A, B and E.

“With our data, they (Certis) were able to get an experimental-use permit to apply what is now MadexXLV,” Nielsen said. “They were really acting proactively.”

So, growers who suspect resistance should incorporate MadexXLV, which functions like an insecticide tank mix, into their rotations, Northfield said. But resistance isn’t the only reason growers might be struggling to keep codling moth in check: Coverage issues and ultraviolet light degradation can both limit virus efficacy.

Teah Smith, an entomologist and pest consultant for Zirkle Fruit Co., said she and her scouts look for apples with codling moth damage weekly and cut them open to see if the larva inside is alive or dead. 

“Damage is not an indication that you have resistance,” she said. “You spray and it takes three to six days for the larva to die. So, I would expect the first and second instars to be dead.”

She likes sampling herself, so she can see where the damaged apples came from and spot a coverage problem if it exists. 

Having a crew rogue out apples with codling moth damage is a good management practice, but it can also camouflage a coverage issue, Northfield said. 

Northfield, Smith and other industry members have recently revamped a Codling Moth Task Force, and they’ve identified the development of resistance management plans as one of their top priorities.

Meanwhile, William Walker, a geneticist with the U.S. Department of Agriculture, has been investigating whether genetic engineering can increase the efficacy of the granulovirus, in light of emerging resistance. He also conducted trials on a new baculovirus species recently detected in South Africa. In trials with his lab colony, he found it to be similarly effective to standard virus products at field rates. 

It’s pending registration, but it could eventually “be another arrow in the quiver for more robust resistance management strategies,” Walker said. 

The prevalence of resistance development remains to be seen. Nielsen has four more populations in her lab that her team is rearing until she has enough to conduct the mortality trials. 

“The bioassay is easy. The hard part is rearing them,” she said. 

The bioassay test involves rearing larvae in little cubes with virus incorporated into their diet. The “discriminating dose” she gives them stems from the protocols developed in Germany and, while in the range of field application, doesn’t directly translate to a field rate, Nielsen said. In the lab or the field, susceptible larva will die after consuming just one droplet, because the live virus will replicate in their mid-gut and stop larval feeding. 

It can take several generations in the lab to build up a sufficient population for these trials. “When they are not exposed to the virus, they can lose the resistance,” she said. With that in mind, even seeing 50 percent mortality in her trials, instead of 100 percent, suggests that the field population she drew from had high levels of resistance. 

Nielsen and Northfield hope to gather a few more suspicious samples this season as well; growers who suspect they have resistance can contact Northfield. 

“We have permits to send your moths to New Jersey,” he said with a laugh.

As for Smith, in light of these findings, she plans to start a drape netting trial as another means of protecting the crop. •