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Flea Beetle Remains A Factor Despite Wet Weather

With the late spring seeding conditions, there may not be much canola seeded and growing across Western Canada in early May, but it won’t be long after the crop emerges that farmers should be looking for signs of flea beetles, says an Agriculture Canada scientist.

Admittedly, cool, and particularly wet conditions, are not conducive for flea beetle populations, says Julie Soroka, an entomologist with Agriculture Canada based at Saskatoon, Sask. But it is a hardy pest that can appear even under less than ideal conditions. And the canola crop is most vulnerable to flea beetle damage in the seedling stage — from cotyledon to the three to four leaf stage.

“Flea beetles prefer warm, dry conditions,” says Soroka, “So cool, wet conditions will put a damper on development of the pest. (But) cool conditions just slow them down, it is more the wet that they can’t handle. So if we have wet conditions as the crop emerges that will affect the pest. But at the same time we can have these micro-climatic conditions, which means if it is wet in one area there may not be many flea beetles, but down the road a ways it could be a bit drier and the beetle will be there. Producers need to be watching.”

CHECK SEEDLINGS

Soroka says because the flea beetle over-winters quite well, fall populations may be a good indicator of flea beetle pressure this spring. If farmers noticed a heavy flea beetle population in the canola crop last fall as they were swathing the crop, that could be a sign of high flea beetle numbers this spring.

If the pests are present they will begin feeding on canola seedlings soon after the plants emerge. As a general guide, the damage threshold for applying pest control measures is when producers observe about 25 per cent of the leaf area has been eaten.

“This is just a visual estimation that perhaps improves with experience,” says Soroka. “It is just a matter of looking at plants and visualizing the area of a whole leaf, and if it looks like about 25 per cent of that area has been eaten, it may be time to apply a treatment.”

Soroka says the pest will likely first appear in crop along the headlands, or could be found on any cruciferous-type plants — volunteer canola and wild mustard, for example — growing near the edge of a field.

ADULT SCAUSEDAMAGE

It is the over-wintered flea beetle adults that have the most economic impact on a new canola crop. There are two primary species — the shiny blue-black crucifer beetle and the yellow striped flea beetle — they are both the same size, and may be found in different regions across the canola growing area of Western Canada, but they behave similarly and cause the same amount of damage.

The adults begin to emerge in spring as soon as daytime temperatures get a few degrees above freezing. This emergence usually happens over a period of two to three weeks but, with cooler conditions, can be drawn out.

The adults lay eggs from mid- May to August, and the second generation of flea beetles will emerge in late summer. The young pests may be visible on the canola crop later in the year, but the plant has matured sufficiently to the point where the pest is not causing an economic impact.

It is the adults feeding on the new seedlings that cause the most damage. Early in the season, under cool conditions, the pest walks or hops from plant to plant, but later as daytime temperatures warm to 20 C or more the pest can fly several kilometers to find food.

ECONOMICS AND TREAT MENT

Depending on pest numbers, flea beetles can reduce canola yield by 10 per cent or more. Feeding on seedling leaves can increase plant mortality, which not only directly reduces yield, but could also produce a thinner crop stand, increasing competition from weeds. Damage done to leaves could delay crop development, cause uneven crop maturity, and can result in increased green seed count. Also the injury to leaves can make plants more vulnerable to plant diseases.

Some seed treatments are effective in reducing the impact of low to moderate flea beetle feeding in the first two to four weeks after emergence. The pest still has to bite the leaf so it can ingest the insecticide, but the treatment should reduce continued feeding.

If the seed treatment did not include a flea beetle control product, or if there is high insect pressure a number of insecticides are available for in-crop application. Again, the recommended threshold to consider treatment is when you observe about 25 per cent leaf damage in the early stages of crop growth — prior to the fourth leaf.

Fields should be scouted during the first two to three weeks after crop emergence. Because the pest usually appears first on the edge or headland of the field, producers are urged to walk well into the field to get a proper representative sampling or assessment of insect damage.

LeeHartisafieldeditorforGrainewsat Calgary.Contacthimat403-592-1964orby emailat [email protected]

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Field Editor

Lee Hart

Lee Hart is editor of Cattleman’s Corner based in Calgary.

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