Why Your Insecticide May Not Be Working as Well as You Would Like

July 31, 2018

Rick Foster & Laura Ingwell, Purdue University
Reprinted from Purdue University's Vegetable Crops Hotline Newsletter

It’s not uncommon for us to get calls from growers who are expressing concern about a particular insecticide product that is not working as well as the growers would like. Often, growers will suggest that Product X is no good or that the target insect has now developed resistance to that particular insecticide. Before we jump to conclusions, we need to consider a number of possible causes for poor performance by an insecticide. Here are some questions to consider if you are not getting the control you would like.

  1. Are you using the right product? Every fall, a group of vegetable entomologists from throughout the Midwest thoroughly revise the insecticide recommendations in the Midwest Vegetable Production Guide (ID56). The products we list in the guide are the ones that we believe work for a particular pest, based either on research trials or through our experience with growers. That’s why we always like to hear from growers when a new product comes along to determine if they are satisfied with the control they received. Sometimes a product will look good in small trials but not so good in large field situations, so that is good information for us to know. Related to this question is “have you identified the pest correctly?” Products can vary greatly on how well they control various pests, so correct identification is critical. Most of you deal with the same insect year after year, so this would not be an issue. However, for some newer or less frequent pests, make sure you are treating for the right pest.
  2. Was your timing right? For many insect pests (for example, corn earworm in sweet corn), timing is critical. Missing the optimal timing for earworms by a couple of days because of a muddy field, can greatly increase the amount of damage when pressure is high. Timing is not as critical for some other insects but growers should be sure to use the economic thresholds that we suggest. We purposely set those conservatively because we know that cleaning up an extremely high population of some pests can be extremely difficult.
  3. How is your water? Many insecticides are subject to degradation by alkaline hydrolysis. Each insecticide has an optimal range for the pH of the water in which it is mixed. Generally speaking, water should be either neutral or slightly acid (pH of 7 or below) to avoid breakdown of the active ingredient. If your water, like much of the water in Indiana, has a pH higher than the optimal range for your insecticide, you should treat the water to lower the pH and improve your control. Likewise, some insecticides are rendered less effective by excessively hard water, another common trait of water in Indiana. Again, for some insecticides, lowering the level of hardness will improve control.
  4. How is your coverage? An old expression on how to get good insect control with insecticides is “the right insecticide at the right place at the right time.” For insecticides that are not systemic (don’t move through the plant), good coverage is essential. We’ve been doing some research on the control we receive from different sprayers or boom types for earworms in sweet corn. We used water sensitive cards to measure the amount of coverage we are getting on the ears and compared that to the level of infestation of earworms in the ears. What we found is that there is a very high correlation between the amount of coverage and the control we received. I encourage all growers to purchase some water sensitive cards and test their sprayers to see what level of coverage they are receiving. If you do that and are not getting the coverage you would like (and your control is not satisfactory), here are some options for improving your coverage:
    1. Increase your gallonage
    2. Increase your pressure
    3. Use drop nozzles for sweet corn
    4. Increase the number of nozzle tips
    5. Change the type of nozzle tips
    6. Drive slower
  5. Is your sprayer properly calibrated? When was the last time you calibrated your sprayer? It should be done at least annually. Nobody likes going through the process of catching water from each of the nozzle tips and doing the math to see if your rates are correct, but it is a necessary evil if you want to get good control. Likewise, if you are spraying a 40 acre field, you are not really spraying 40 acres. You have to account for fence rows and field margins when calculating how much area you are spraying.
  6. Is your sprayer in good working order? If you calibrate your sprayer, you will quickly find out if some nozzle tips are clogged or worn. There should be less than 5-10 percent variation between the output of different nozzles. If the variation is greater than that, the nozzles should be either cleaned or replaced. Have you checked all your hoses to make sure that there are no leaks? Insecticide leaking from a hose is not going to kill any insects and that means you are putting on a lower rate than you intended on the plants.
  7. What time of day are you spraying? Most insecticides are broken down by heat and sunlight so the optimal time to spray insecticides is late evening or at night when temperatures are dropping and sunlight is fading. Many insect pests are nocturnal, so spraying right before they become active will increase the control of that first night. Also, when spraying in the heat of the day, some of the insecticide will volatilize before it reaches the plant, again resulting in a lower rate of residue on the plant. An added benefit of spraying in the evening/night is that pollinators will have left the field and any harm done to them will be minimized.
  8. What are you mixing with your insecticide? Some insecticide/fungicide combinations are not compatible. Be sure to check the labels before mixing. Also, check the label to see if the manufacturer recommends any adjuvants be mixed with the insecticide.
  9. What is the weather like? Weather can be an important factor in the effectiveness of an insecticide application. For example, the pyrethroid insecticides (Warrior®, Capture®, Baythroid®, Mustang Maxx®, etc.) tend to break down more quickly when temperatures are high, upper 80s or 90s. If you are experiencing decreased control with pyrethroids during warm period, consider switching to a non-pyrethroid option. In contrast, products that need to consumed by the pest, such as Bt products, tend to be more effective when temperatures are warmer because the insects are more active and consuming more insecticide as they feed. Rainfall can also be a big factor. Insecticides vary greatly in how well they adhere to the plant after a rainfall. As a general rule of thumb, if the insecticide completely dries on the plant before rain occurs, a light rain will have little effect on the residues. The heavier the rain event, of course, the more insecticide will be washed off the plant. Ideally, you would want to spray after the rain, but that often is not possible because the fields are too muddy for your sprayer to pass through the field.
  10. Is the target pest resistant to the insecticide? We purposely listed this possibility last because, while resistance is a real and increasing problem, it is usually not the reason for poor control. In recent years, corn earworms developed resistance to pyrethroid insecticides that had provided excellent control for many years. We were able to confirm that with laboratory studies, but growers should not jump to conclusions about resistance based on a single case of poor control.

Read the original article and view addition articles on the Vegetable Crops Hotline newsletter.