When we talk about insect control we start to appreciate that it is a complex problem. There are many factors, like economic threshold, optimum application timing, pre-harvest interval, effect on beneficial insects, and making the crop less of a target for pests.
Most of my discussions with farmers are focused on spraying as a direct control measure. Don’t overlook everything else that must be done as part of an integrated pest management program before taking the last option of using in-crop insecticides.
Here is a list of things to consider as part of your insect management plant.
1. Acceptable population levels
Before you spray, make sure you know the acceptable level of the pest population — a few pests will always be present.
Make regular field observations to monitor the pest population and properly identify most of the insects.
2. Farming practices
Are you presenting a smorgasbord of food with no barriers to the incoming pest invasion or do you have prevention program in place that makes the neighbours’ fields an easier target?
Making the crop less of a target should be at least one goal when managing insects.
I would like to see more use of trap crops in insect management.
A trap crop may be a strip of alfalfa seeded on the edges a canola field as a trap crop for lygus bugs. Instead of spraying the canola, monitor the alfalfa strips and spray it for insects.
Alfalfa strips may also be used as a breeding ground for beneficial insects that can help keep pest populations in check.
Healthy crops may be another part of the plan. An observation from the field is that crops under stress take a bigger hit from pest insects than those that are not showing signs of stress. Higher insect counts are more likely in areas of the field that are growing poorly than in heavy crop. Does good plant nutrition have an effect on the crops response to insect pest attack?
We do know that plant stresses are cumulative. Plants that are already suffering from adverse weather, lack of nutrition and then added insect damage the plants will have a hard time recovering or growing through it.
We believe that if canola is not getting enough boron it may cause terminal bud death and will try to branch out around that bud. The question might be does a temporal nutrient deficiency plus a few too many lygus bugs at budding stage have a combine effect that causes a greater problem?
3. Economic thresholds
Economic thresholds for insect control change based on current crop value and anticipated control cost. A higher value crop requires fewer pest insects to make it worthwhile to apply a control measure that reduces the potential damage to the crop.
Economic threshold charts are available online. Look for updated versions with current crop prices.
Scout fields at regular intervals and check that populations are building if planning to spray insecticide. Consistent checks in the same areas and at similar time of day will help. Windy days may give you different counts than on calmer days, temperature is a factor when scouting for insects, and insects are usually less active at cooler temperatures. The key is to control insects when they have reached at economic threshold counts on average counts across the field.
When scouting for insects don’t miss the forest for the trees. It is very easy to be focused on looking for a specific insect pest when using the sweep nets or walking in the field. You might miss other insect activity that is going on around. Other insects that are not pest are likely beneficial insects helping with the whole crop ecosystem — pollinators, or insect predators that hunt for and prey on other insects.
5. Application timing
Optimum application timing of insecticides can make the difference between money well spent and crisis averted or basically a revenge spray that kills the pest but long after the damage has been done.
One discussion regarding application timing is about spraying an insecticide on canola at budding, especially for lygus bugs. Our experience seems to provide evidence that if lygus bugs are present at the bud stage and the canola plants struggle to produce proper flower buds, spraying an insecticide does relieve some of the pressure on the plant so that it can produce flower buds.
The key is to check canola at the bud stage and determine if budding and flowering is progressing properly and, if not, what might be holding it back.
We do see quite a few farmers doing prophylactic spraying for flea beetles in canola who maintain they are also controlling other insect pests like lygus bugs and diamond back larvae early. The question is how many insect pest predators also get wiped out when spraying for pest insects that may or may not be there that early?
6. Population data
Forecast Maps and Monitoring Networks provide calculated information on many insect pests based on pheromone bait traps, insect count surveys, and early warning monitoring sites. Check this information as standard practice to prepare for anticipated problems along with your own in-field monitoring program.
7. Pre-harvest interval
In 2012, many farmers checked their canola for swathing stage and realized they had a bertha armyworm outbreak. We had several calls from farmers in this situation, wondering what insecticide they could use, or if they should even be spraying at all, that late in the season.
The pre-harvest interval is the numbers of days between product application and either swathing or straight cutting the crop.
With bertha armyworms the damage can be pretty severe and not controlling them even for a few days will cost you some yield. If you didn’t realize you had significant berthas up to a week before the crop is ready to swath, you’ve have lost yield potential.
If you think you’re more than seven days from swathing or straight cutting the crop you have insecticide options that will fit in the pre-harvest interval, check the label to make sure.
In 2012 monitoring intelligence at the end of May reported high numbers of leaf hoppers moving into the Prairies from the Midwestern states — travelling on wind currents and settling down onto canola and cereal crops with the potential to spread aster yellows disease.
We did see aster yellows in many fields but many farmers were not aware of what was happening to the plants in the field until much later, when infected canola plants were producing pods that looked like deflated bladders instead of seed filled pods. A guaranteed economic return from spraying for leafhoppers is doubtful, even vegetable farmers who spray insecticide on a weekly basis for leaf hoppers will still get some aster yellows infection. There is a lot of work done to prepare forecast maps and provide information to the pest monitoring networks, use them as a resource to plan ahead.
Make an insect management plan that assess factors like economic threshold, optimum application timing, pre-harvest interval, effect on beneficial insects, and making the crop less of a target for pests to help take the uncertainty out of your decision making. †