There’s a worm in my apple: Failure of an insecticide to control an insect can be attributed to many things. When control failures occur we better ask ourselves some hard questions. Aside from assuring yourself that it wasn’t one of the usual suspects such as equipment failure (nozzle or strainer plugged) , environmental conditions (such as wind, wash off from rain), ineffective or low labeled rates, incorrect product use (no efficacy against target pest), what else might have allowed this to slip in?
The possibility of resistance in broad spectrum or older insecticide chemistry is always a looming question.
This is especially true for pyrethroids and organophosphates in combination with pests such as codling moth. Codling moth has, over the past 50 years, traditionally been a migrant pest, often moving into orchards from abandoned orchards. However, with the shift in insecticide availability (loss of insecticides such as Calypso, loss of Lorsban use at petal fall), we are seeing an increase in the codling moth population with more orchards finding worms in their fruit, some for the first time.
Like the obliquebanded leafroller (OBLR), the insect can become an endemic pest. Once it begins to reside in the orchard, it will be exposed to insecticides year after year. This constant exposure to repeated use of an active ingredient in the product is the ‘selection pressure’ that refines the population, selecting individuals that have become less sensitive to the insecticides used in the orchard. We call this insecticide resistance.
The organophosphate class of insecticides, including Guthion (azinphos-methyl) and Imidan (phosmet) have been used since the 1960’s, for over 50 years, to manage the codling moth. The development of resistance by codling moth to Imidan is likely if its reoccurring use has been for Plum Curculio and or OBLR management during mid-summer. Consistent use during the past ten years or more increasing resistance potential. Those specific timings would provide some level of control of CM while providing various rates of residual exposure. Low rates of residual increase the selection for resistance during the early and mid-summer generations of the pest. A scenario, such as the switch from Guthion to Imidan in season long program use would affect the same target site (nerve receptor sites for ACh, acetylcholinesterase) within the insect to contribute to resistance.
In the Hudson Valley to date I don’t believe resistant strains are widespread, even though we are hearing of increasing reports of CM damage to tree fruit. The broad availability and use of insecticides that include the different IRAC classes of active ingredients for plum curculio from PF to 2nd cover and mid-summer management of OBLR, would reduce region-wide resistance of CM to any one specific insecticide class. We have seen the use of these materials to include Avaunt, Exirel, Calypso, Carbaryl, pre-mixes including pyrethroids or pyrethroids alone. Insecticides of various classes for the overwintering OBLR used at PF would likely impact CM to a lesser degree when used at petal fall timing.
There haven’t been recent studies to detect phosmet resistance to CM in NY that I’m aware of. In Michigan, a study in 2008 detected a 7-8 fold level of resistance in orchard site specific CM populations to codling moth (see abstract from work done by John Wise in Mich. State below). However, no resistance to acetamiprid (Assail) and spinosad (Delegate, Spintor, Entrust) was detected in the study.
That said, it’s very likely we have codling moth populations resistant to older insecticide classes, including pyrethroids and OP’s in orchards throughout the Northeast.
Insecticide rotation: I would suggest the use of specific materials for CM be employed during 1st-2nd cover and again in mid-July, when model predictions for larva emergence are called for AND in orchards with reoccurring fruit injury from CM. The use of Assail to manage apple maggot over the next few weeks during CM larval emergence is a good option as it’s one of the better materials against both CM & AM. Next season, the use of Delegate, Altacor at 1st – 2nd cover would reduce the resistance potential while picking up overwintering OBLR. Mating disruption and granulosis virus are also good choices in conventional and organic production systems. Rotating classes for EACH GENERATION not each spray during a generation) will reduce the potential for insecticide resistance.
Pest Manag Sci. 2008 Sep;64(9):881-90. doi: 10.1002/ps.1576.Resistance of codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), larvae in Michigan to insecticides with different modes of action and the impact on field residual activity.
Mota-Sanchez D1, Wise JC, Poppen RV, Gut LJ, Hollingworth RM.
The codling moth is one of the principal pests of apple in the world. Resistance monitoring is crucial to the effective management of resistance in codling moth. Three populations of codling moth in neonate larvae were evaluated for resistance to seven insecticides via diet bioassays, and compared with a susceptible population. In addition, apple plots were treated with labeled field rate doses of four insecticides. Treated fruit were exposed to neonate larvae of two populations from commercial orchards.
RESULTS: Two populations of codling moth expressed two- and five fold resistance to azinphos-methyl, seven- and eight fold resistance to phosmet, six- and tenfold resistance to lambda-cyhalothrin, 14- and 16-fold resistance to methoxyfenozide and sixfold resistance to indoxacarb, but no resistance to acetamiprid and spinosad. The impact of the resistance to azinphos-methyl, measured as fruit damage, increased as the insecticide residues aged in the field. In contrast, fruit damage in methoxyfenozide- and lambda-cyhalothrin-treated fruit was observed earlier for resistant codling moth. No differences in efficacy were found for acetamiprid.CONCLUSIONS: Broad-spectrum insecticide resistance was detected for codling moth. Resistance to azinphos-methyl, lambda-cyhalothrin and methoxyfenozide was associated with reduced residual activity in the field. Broad-spectrum resistance presents serious problems for management of the codling moth in Michigan.