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Why is grape berry moth so difficult to manage?

Grape berry moth (GBM), Paralobesia viteana, has been a difficult pest to manage for a very long time, especially in areas with consistently high populations of this insect. In the past, there have been some broad spectrum pesticides that gave adequate control, but they came with their own set of problems including a high level of non-specificity (i.e. they killed beneficial insects and mites along with the pests). Although there are rumours of an extension of the registration for the broad spectrum pesticide azinphos-methyl (Guthion), the logistics of working around a 28 day re-entry interval (REI) for grapes are problematic to say the least.

Although there are several products that are "in the works" for GBM, there's not much point discussing them in detail until they are registered. Newer products, as they become available, come from a wide range of chemical families. Many, if not most, of the new products are not rapid contact poisons; rather, they must be ingested to work or they may not produce rapid knockdown of the pest. If you saw the video presented by Dr. Rufus Isaacs at the grape sessions of the OFVC in February, you were presented with a vivid demonstration of why coverage is so critical with non-contact insecticides. As the video showed, the larval GBM do not even eat the grape skin where they enter the fruit; instead they chew off bits of the grape skin and toss them away. So, for those products that must be ingested, the mouthparts of each larva must come in contact with some of the product at exactly the right time at the tiny site where they chew their entrance into a berry.

Some products that are still available for GBM management are seen by some to be ineffective. Folk wisdom from some quarters has it that registered pyrethroids, carbamates, and organophosphates are all ineffective! Does this mean that resistance to pesticides by GBM is rampant? There's no proof of that in our area and the most recent survey for GBM resistance in Niagara (Pree et al. 1998) found no resistance to Guthion. Of course, absence of proof is not proof of absence, but there are other possibilities for poor control that should be considered before claiming resistance is the problem. The other side of that coin is that a population can become resistant if good resistance management practices are not followed. That will always be the case, so rotation of control products by pesticide family or mode of action will always be critical.

Why else might some believe that so many products are ineffective against GBM? Some basic application issues may at times be the reason (see below) but there are also product-specific considerations that should not be ignored. For instance, the insecticidal activity of Imidan (phosmet) is reduced if spray water is above pH 7.0; use a buffer if necessary to get pH in the range of 5.5 to 6.5. Also, the activity of Diazinon is reduced when temperatures are below 20°C while pyrethroids are less effective in hot temperatures; labels of pyrethroids indicate reduced efficacy if daytime highs are above 25°C and suggest that you spray in early morning or in the evening when temperatures are cooler. However, if regular daytime highs are well above 25°C, pyrethroids will be efficacious for only a few days at best regardless of the temperature at application.

Such seemingly simple and basic considerations as ensuring good coverage through canopy management and adequate water volumes cannot be abandoned even if you are upgrading to a new sprayer system or have a desire to cover more acres of vineyard. Speaking of which, Dr. Andrew Landers provided much food for thought on sprayer adjustment and accurate delivery of pesticides in his presentation at the OFVC grape session. Dr. Landers had a lot of take home messages in his presentation but one which must always be repeated is to make sure you are getting your products to the target without wasting your time and money.

An important factor in dealing with GBM is that its basic biology is in conflict with your best attempts at managing it. Adult emergence in the spring is often extended for up to six weeks, ensuring that overlapping generations will occur almost every season. Coordinating spray timing with peak egg hatch or peak egg laying is thus very difficult.

Of course, if your pest management technique doesn't target eggs or hatching larvae, but instead affects the behaviour of adult moths, you would have to be doing it all season. That's where mating disruption (MD) excels - once the dispensers ("twist-ties") are put in the vineyard, MD continues for the entire season. MD for GBM has risen rapidly in acreage in the last two years and it alleviates many of the management issues of conventional pesticides.

Is MD perfect and the solution to everyone's GBM problems? Of course not, there are limitations to all technologies, but MD generally works well for GBM, requiring a minimum vineyard size of 5 acres (2.5 ha) to be effective in most cases. Pest pressure must be low to moderate for MD to work, so knowledge of the history of GBM problems on a site-specific basis is needed before starting an MD program. Transition years using both MD and insecticides can often help bring a high pressure site down to a level where MD can work alone. MD works best if the pheromone dispensers are hung in the vineyard before first flight of the moths in the spring. MD is not a "clean-up" approach and cannot help after moths have mated and are laying eggs in the vineyard. Always plan an MD program in consultation with your monitoring service in advance of the beginning of the growing season.