Most growers continue to rely on insecticides as their principal control tool for this pest complex, but more and more growers are also adding sex pheromone mating disruption to their management toolbox.
Despite the loss of some valuable insecticides due to the Food Quality Protection Act (FQPA) and the development of insecticide resistance to a number of other remaining products, the toolbox for the control of the internal fruit-feeding complex continues to expand each year. Among the many new tools available to control CM is a naturally occurring virus that was identified back in 1964 in Mexico on infected CM larvae. Because of its high selectivity toward this pest, it is called the codling moth granulovirus (CpGV). It shows some activity on a couple of closely related species (e.g., OFM), but it is noninfectious toward beneficial insects, fish, wildlife, livestock, and humans.
Mode of action
Each CpGV particle is naturally microencapsulated within a protein occlusion body (OB) that protects it to some degree from degradation. These viral OBs are extremely small, 400 by 200 nanonmeters (i.e., 4,000 OBs placed end to end are approximately 1/16 inch). Depending on the product, a single ounce of the aqueous suspension concentrate can contain more than one to three trillion OBs. In order for the virus to be effective, the tiny particles must be ingested by the larva--there is no contact activity with CpGV.
It only takes a couple of these OBs to cause death in a young larva. Once the larva ingests the virus, the OBs are dissolved in the alkaline gut of the larva, rapidly releasing the viral particles. The virus rapidly penetrates the gut lining, causing the virus to replicate numerous copies of itself, which then rapidly spread to other organs within the larva. This multiplication causes the larva to stop feeding within a few days, becoming sluggish and discolored as the virus moves throughout the body of the insect. Upon death, the larvae "melts," spreading billions of the viral OBs that can be ingested by other CM larvae. Each OB is capable of causing a new infection within other newly hatched larvae.
In Pennsylvania, four products are currently available for use by fruit growers: Cyd-X, Cyd-X HP, Madex HP (all from Certis, USA), and Carpovirusine (Arysta LifeSciences, Inc.). Madex HP is also registered for the control of Oriental fruit moth. All products can be used right up to the day of harvest and have a 4-hour reentry window. The products should be refrigerated until use because warm temperatures cause degradation of the OBs. Also, these products are certified for use in organic orchards.
We have been researching both products at the Penn State Fruit Research and Extension Center during the last few years and have achieved much success in substantially reducing CM populations, especially where CpGV was integrated with some form of sex pheromone mating disruption for CM. There are a number of opportunities for using CpGV in a CM management program.
Before using a CpGV product, however, there are a number of important points to understand: (1) the virus must be ingested by the larva, thus timing and coverage are extremely critical; (2) the virus breaks down rapidly in an orchard environment due to both UV rays from the sun and rainfall, thus spray intervals should not be stretched for more than 7 to 9 days; and (3) the feeding larva causes some injury to the fruit, commonly referred to as a "sting"--injury less than 1 to 2 mm in depth--before the virus eventually kills the larva.
Since CpGV is most active against young larvae and these larvae usually penetrate the fruit within 24 hours of hatching from the egg, it is very important to have the virus present when egg hatch begins (approximately 230 to 250 degree days [DD] following biofix--first sustained adult capture in a sex pheromone trap). If CpGV is intended as the primary control tactic for CM, then the first application should be timed to coincide with the beginning of egg hatch. Depending on the length of the egg hatch period, normally a total of three to five applications each spaced about 7 to 9 days apart will be necessary to cover this time frame.
Recently, in some apple orchards in Pennsylvania, we have observed the egg hatch period for CM to extend over a longer period of time than what is normally predicted by the CM developmental model. Under these conditions, it may be prudent to apply an insecticide with ovicidal activity (i.e., against the eggs, Esteem, Intrepid, or Rimon) at approximately 75 to 150 DD, then start the CpGV applications at about 300 to 350 DD, and repeat the applications every 7 to 9 days or approximately every 125 to 150 DD following this initial application. Since the virus rapidly breaks down in the orchard environment, it is our experience that frequent applications of a lower rate are better than high rates applied at longer spray intervals.
As stated above, the CpGV must replicate itself within the larva in order to be effective, thus allowing the larva to continue to feed for a few days and causing some shallow feeding damage ("stings") to a fruit. If growers are trying to decide when to use a CpGV product, they may want to restrict their use of a CpGV product to the first generation. If stings or even some deep entries do occur in the small fruits during June from the first-generation larvae, these fruit often fall from the tree or can be thinned off. In addition, at this time of the season, the fruit on the tree are still small and canopy volume is still not complete, thus allowing more thorough coverage of the fruit.
CpGV products are compatible with most fungicides and insecticides sprayed on apples. However, since CpGV is sensitive to high alkaline conditions, it should not be mixed with copper fungicides or lime sulfur. In addition, it is recommended to use a buffer to neutralize the spray mix if the pH is above 9 or below 5. Also, Dr. Larry Gut, entomologist at Michigan State University, has cautioned Michigan growers to avoid tank mixing CpGV with neonicotinoid insecticides such as Assail, since these compounds have some anti-feeding activity which may interfere with the larva ingesting the virus.
Since UV light can rapidly break down the virus particles, it is also recommended that growers avoid applying the virus during periods of intense sunlight conditions. Also, if rain is forecast in the immediate future, try to wait until after the rain period to make the application.
Many growers in Pennsylvania commonly make their pesticide applications using the alternate row middle (ARM) method of spraying. We have used CpGV successfully with ARM sprays, but the studies have always been conducted with sex pheromone mating disruption for CM as a basic component of the program. Thus, here are recommendations for applying CpGV using the ARM approach:
- Since the virus must be consumed, thorough coverage is critical. Thus, ARM sprays must provide some coverage on the unsprayed side of the trees.
- Depending on the size of the tree, water volumes of at least 50 (trees 6 to 10 feet in height) to 100 gallons per acre (GPA) (trees larger than 10 feet in height) should be used.
- Dependent on pest pressure and weather conditions, ARM intervals should not stretch beyond 5 to 7 days between sprays.
- This method of applying CpGV should only be used in conjunction with some form of CM mating disruption.
Recommended Use Options for CpGV Products within Pennsylvania Apple Orchards
- Make the first application at the beginning of egg hatch (i.e., approximately 230 to 250 DD after biofix).
- Use an ovicidal insecticide at approximately 75 to 150 DD, then begin virus applications at roughly 300 to 350 DD.
- Repeat applications every 7 or 9 days or roughly every 125 to 150 DD.
- Use a higher rate of CpGV for the first application.
- Repeat applications at lower rates for subsequent applications.
- Apply three to five applications for the first brood depending on the length of the adult flight and egg hatch period.
- Use primarily for first-brood CM control.
- For more effective control, combine with CM mating disruption, especially where CM populations are high and/or fruit injury from CM was present last season.