to the weekly Vegetable Pest Message
from the University of Connecticut Cooperative Extension System
Week of July 6th Vegetable Pest Message
Click here for previous Pest Messages
Welcome to the University of Connecticut Cooperative Extension System’s Vegetable Pest Message.
This week’s message will cover:
• Corn earworms
• European corn borer
• Mixing or alternating between insecticides
• Pepper maggots
• Early blight not late blight
• Powdery mildew on summer squash
Pheromone traps in Wallingford, Berlin, Rocky Hill and Glastonbury all failed to capture CEW moths in the last few days so those sites are not spraying fresh silking corn at this time. The only site where we found CEW moths this week was in Canton which captured an average of 1.5 moths per night, which dictates a 4-day schedule. Traps on Long Island increased substantially this week so that most farms there are on a 4-day schedule on fresh silking corn, so watch your traps closely as I expect the numbers to increase in the next few days, especially if the winds switch and come from the southwest. Here are the CEW trap thresholds:
Moths/night spray interval
0-0.2 No spray
0.2-0.5 6 day schedule
0.5-1 5 day schedule
1-13 4 day schedule
>13 3 day schedule
European corn borer
Pheromone traps in Northford, Berlin and New Hartford failed to capture borer moths again this week indicating that we are still between generations. Most fields we scouted were free or almost free of borers, however, one field in Wallingford was still above the 15% infested plant threshold at pre-tassel and required treatment and another field in W. Suffield had 12% of the plants infested. No FAW larvae, moths or damage were detected.
Mixing or alternating between insecticides
It seems that sales reps have been pushing a new product that is a pre-mix of Coragen and Warrior and a couple of growers asked me about it this week. Although it seems like mixing insecticides might make for a good resistance management program, in truth, you are probably far better off to alternate or rotate between insecticide products than to mix two or more products together in each application. Adding a second insecticide simply increases the expense of each application, makes it harder to find a new insecticide family to use on the next generation, wipes out more natural enemies, and increases other hidden costs or risks associated with using pesticides such as environmental pollutions problems, applicator safety, and multiple reentry and day-to-harvest restrictions.
The Insecticide Resistance Action Committee (IRAC) has outlined several basic rules or assumptions that must hold true to extend the useful life of insecticides when mixed.
Getting extended use from insecticides by mixing assumes that:
1. some susceptible individuals from the pest population are spared
2. that each pesticide in the mix has a different mode-of action,
3. and the same length of effectiveness,
4. that resistant individuals are rare, and
5. that resistance is passed on to offspring by a single recessive gene.
The problem with the mixing theory is that these assumptions are unlikely to hold up in the field. In reality, few people would like to bear the expense of always using multiple products in their tank. So, we seldom start mixing products before we begin to have pest control failure problems with the first product. In this case, Warrior was on the market for many years before Coragen, and resistance to Warrior and other synthetic pyrethroids is already fairly widespread, so you can assume there are resistant individuals out there in your field already. According to resistance management theory, it is too late to start mixing at this point because resistant individuals would no longer be rare (assumption #4). This would accelerate the development of resistance to the second product rather than retard it due to excessive use and selection pressure.
It is also unlikely that products with different modes-of-action would have a similar residual period of effective activity. For instance, in this case Warrior would have a much shorter residual period that Coragen, which is active for up to 3 weeks on some cabbage pests, so assumption # 3 will rarely if ever be met. That means there will be times when pests will be exposed to a single insecticide rather than a mix and the mixing theory breaks down again. Finally, if multiple genes are involved in the inheritance of resistance, then mixing insecticides can actual hasten the development of resistance instead of delaying it.
On the other hand, rotating between insecticides with different modes-of-action, and only using one material at a time, fits right into the IPM and resistance management philosophy of trying to minimize pesticide use. Using one insecticide at a time, instead of two or more in each application, lowers the related expenses, costs and risks associated with chemical pest control.
We found pepper maggot stings and eggs on cherry pepper fruit at Rocky Hill and E. Hartford this week, which signals the start of the pepper maggot flight. Stings show up as slightly indented “dimples” on the glossy cherry pepper fruit. Dimethoate or Orthene should be used at 10 day intervals at farms that have a confirmed population of this pest. Organic growers should use OMRI-approved GF-120 Fruit Fly Bait or try the new OMRI-approved cutworm and fly bait Seduce. We have one organic farm in W. Suffield that will be trying Seduce this season and I’ll try to let you know how they make out. The most effective control of this pest is achieved with perimeter trap cropping, where hot cherry peppers are planted and sprayed in the outside row all around the field and the rest of the crop is left unsprayed. We had good results using a combination of GF-120 and perimeter trap cropping last year in W. Suffield. Occasionally PM attacks eggplant and neither dimethoate nor Orthene are registered for that crop. You can however use perimeter trap cropping by planting hot cherry peppers around the eggplant and using a shielded spray on just the cherry peppers, or you could try GF-120 or Seduce.
Early blight not late blight
Each year since the late blight epidemic of 2009, we get a few false alarms where a grower thinks they have late blight in their tomato or potato field. On potatoes it usually turns out to be potato leaf hopper “hopperburn” which turns the leaves black. On tomatoes, it usually turns out to be either bacterial canker, Septoria leaf spot or early blight. This week I checked out a suspected late blight infection complaint that turned out to be a bad case of early blight, with all the classic ½-inch oval lesions and yellow lower leaves that are normally associated with early blight. However, since we have had one confirmed case of late blight this year in CT, and this week there were new cases in Long Island, MA and ME, all growers should be putting on regular fungicide cover sprays at this time. Growers should be protecting their tomato or potato plantings with applications of a dithane- or Bravo type products on a 7 to 10 day interval. Organic growers should use copper products approved by their certifier. There have been no new confirmed cases of late blight in CT since the first incidence reported two weeks ago.
Powdery mildew on summer squash
We are finding PM on green and yellow summer squash. Look at the underside of older leaves for small, round, white colonies of the fungus. Begin spraying summer squash when you find the first sign of the disease and treat every 10 days until you get near the final harvest for each planting. The systemic fungicides are the most effective at controlling this disease because it starts on the bottom of the leaves where it is hard to get good coverage with any sprayer. Products with 0 dh restrictions such as Pristine (group 7) and Procure or Rally (group 3) will simplify harvest operations. Only a single application of each family or group of systemic fungicide should be used on each planting and a protectant such as Bravo or sulfur should be mixed with the systemic to help prevent resistance. Sulfur can also be used to slow the spread of PM and is the most effective fungicide available on organic farms, but we also use it on conventional farms once we have used a single shot of group 7 and 3 products.
That’s all for this week. This message will next be updated on Friday afternoon July 13.
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