How to assess leaf area loss from flea beetles

The action threshold for flea beetles in canola in Canada is an average leaf area loss of 25 per cent or more. To make that assessment, follow these three steps:

flea beetle defoliation damage on canola plants
This graphic shows what various levels of leaf area loss look like.
  • Figure out how to estimate 25 per cent loss on one plant. Use the graphic below as a guide. This isn’t a perfect science, so do your best. An important note when assessing loss: A plant that has serious (killing) levels of stem feeding can be counted as 100 per cent leaf area loss.
  • Estimate the average leaf area loss for 10 plants in the immediate area. Toss your plant counting hoop or anything (like a wrench) to find a random scout spot. Assess 10 plants closest to where it lands, and compute the average leaf area loss per plant. Note that flea beetles will often enter a field from the edges, so edges will give skewed results. Walk toward the middle of the field to find a start point for a first assessment.
  • Check a few places throughout the field. The usual scouting technique is to make assessments at five points, in a “W” pattern. Check 10 plants at each point of the “W”. Average the results. Is that average over 25 per cent?

If average loss is over 25 per cent, consider spraying if:

Flea beetle management
  • Flea beetles are still feeding. If it looks like populations are dwindling, an application may not be necessary. If flea beetles are slow and lethargic, it may mean they have ingested seed treatment insecticide and are no longer feeding. If new tissue and young leaves are free of damage, beetles are no longer feeding, and the seed treatments have done their job. See a more detailed description of these steps. Note that time of day and weather can influence flea beetle activity. On rainy days, for example, flea beetles will usually take cover, so rain will slow or even stop feeding for the time being. Rain can also help the crop more quickly recover. Cool and windy weather will drive the flea beetles lower to the ground and may result in stem feeding and/or reduced feeding. Insects are completely temperature driven. Warm weather may speed up beetle feeding more than crop growth increases, so scout more frequently in warm weather.
  • Crop is still at high-risk stage. After the four-leaf stage (four true leaves), the threat is likely over because the crop usually has enough plant material to feed flea beetles without compromising growth, and the plants can compensate for feeding better by this stage. If the crop is uneven (some plants are at the four-leaf stage and some are earlier), keep scouting until most of the crop has at least three or four true leaves. Also keep scouting if the crop seems to have stopped advancing due to environmental stress, such as drought. If new material and leaves are emerging with little or no damage, beetle feeding has ended and no foliar application is warranted.
  • Plant counts are low. With a thin stand of four or fewer plants per square foot, growers can’t afford to lose any plants. In that situation, they may want to take action quickly as damage approaches 25 per cent leaf area loss.

A crop under major stress: A thin crop, struggling to grow in hot weather and dry soil conditions may have all three of the risk factors listed above. And since seed treatment insecticide may be wearing out, this crop will require frequent scouting and a potential foliar insecticide. Stressed plants also tolerate less flea beetle feeding than canola growing under more ideal conditions. In these high stress situations, consider spraying sooner than the action threshold.

Tips to improve spray results

Low efficacy for in-crop insecticide applications might be a bigger deal than people realize.

  • Consider the weather. Cool days: Insecticide will be effective on cooler days as long as the flea beetles are active. Malathion is the only product that requires a minimum temperature of 18-20°C. Other product labels say to apply when flea beetles are active, but to avoid the warmest parts of the day. Hot days: Pyrethroids have restrictions for application in higher temperatures. This group includes the most commonly used products. The label for pyrethroid Decis (active ingredient deltamethrin), for example, says: “DO NOT spray under a strong temperature inversion, or when temperature exceeds 25°C as this will result in a reduction in control.” Research from 1970s (Harris and Kinoshita, 1977) showed that pyrethroids were 2.6 times more potent at 15°C than at 32°C. Cool and wet: If conditions are cool AND wet, don’t bother spraying anything. Flea beetles don’t like rain, and will take cover in the soil and leaf litter. Product labels also say not to spray if rain is likely within one hour.
  • Herbicide nozzles are not ideal. Tank mixing with herbicide also reduces efficacy because low-drift herbicide nozzles, which are a good practice for some herbicides, produce a coarse spray droplet that may not provide efficient coverage or flea beetle contact for higher product efficacy. As sprayer specialist Tom Wolf says, “the likelihood of a flea beetle ingesting leaf tissue at the spot where the insecticide hit is low, and gets lower the larger the droplets.” If farmers can improve insecticide efficacy, they won’t have to spray as often.