Crop Production News - 2018
By James Tansey PhD, Saskatchewan Provincial Insect/Pest Specialist, Regina
With the late seeding seen throughout much of Saskatchewan this year, a recurring question has been, “How will flea beetle populations respond?” Because older seedlings can better withstand flea beetle damage, early seeding can be a way to mitigate flea beetle pressure. This year’s late seeding means that seedling emergence and the bulk of flea beetle emergence could coincide in some regions. Forecasted warm, dry weather could also mean highly favourable conditions for flea beetle feeding.
There are two economically important flea beetle species in Saskatchewan: The crucifer flea beetle (Phyllotreta cruciferae) and the striped flea beetle (P. striolata). Both species overwinter as adults in shelterbelts under the previous year’s plant material. Not all flea beetle activity begins at the same time. Some members of populations begin to emerge early, some emerge late. Emergence can extend over weeks. Crucifer flea beetle emergence is highest when soil temperatures reach 15 C. Striped flea beetles can emerge four weeks earlier. Temperature is the big driver for emergence and it’s been warm lately.
These insects are attracted to plants in the mustard family and will readily feed on flixweed, volunteer canola, wild mustard or a canola crop if it’s available. Crop damage occurs when numbers are high. To protect canola seedlings from flea beetle damage, much of the canola seed planted in Saskatchewan is treated with the neonicotinoids: thiamethoxam or clothianidin. Both of these compounds are effective for reducing crucifer flea beetle damage. However, striped flea beetles are less sensitive to these substances. Seed treatments that contain cyantraniliprole and mixes of cyantraniliprole and neonicotinoids are also available. Because the toxin(s) associated with seed treatments needs to be eaten by the insects to take effect, damage can still occur, particularly if beetle populations are high.
Regardless of the product used, it’s still important to scout, especially when seedlings are young. Flea beetle feeding results in characteristic “shot-hole” damage. Their feeding can devastate cotyledons, first true leaves, petioles and stems, and reduce yields. Seedlings are most sensitive to flea beetle damage in the first two weeks after emergence and should be scouted frequently during this period. If 25 per cent defoliation has occurred to cotyledons across the field and beetles are still numerous and actively feeding, the action threshold has been reached and foliar application of insecticide should be considered. Damage can accumulate very quickly. When scouting, keep in mind that these beetles aggregate. High numbers on one spot may not be seen elsewhere in the field. This is why multiple locations should be scouted. Examining 20 plants per site at 10 sites throughout the field is recommended. If cotyledon damage has occurred but no damage to the first true leaf is apparent, flea beetle feeding may be waning and spraying may be unnecessary. Once the 3-to-4-leaf stage is reached, plants are much more tolerant of feeding.