Insects

Table of contents

    Important tips for best management

    • Use seed treatments to control flea beetles early in the season. Current neonicotinoid seed treatments are effective but require ingestion of some plant tissue, so rapid emergence and early growth is still important to offset this feeding. If defoliation exceeds 25% of leaf area consider a foliar insecticide application.
    • A higher frequency of crop scouting may be warranted if flea beetle populations include a high proportion of striped flea beetles (Phyllotreta striolata (F.)) relative to the crucifer species (Phyllotreta cruciferae (Goeze)), as research indicates they may be more tolerant of neonicitinoid seed treatments leading to greater feeding damage from a similar population density.
    • Insect feeding can undo any effort to establish a dense and robust canola stand. Flea beetles and cutworms are the most common early season insect pests. Wireworms and June beetle larvae can also feed on canola seedlings, but tend to be more prevalent on fields previously in pasture or forage production. Diamondback moth larvae feed on seedlings, but rarely arrive at this early stage in numbers sufficient to cause significant plant mortality.
    • Canola can branch out and recover from stands thinned by insects, but early insect feeding will reduce crop biomass and delay maturity, increasing the risk of lower yield and quality. Scout fields frequently from germination and emergence through the early rosette stage for feeding damage on leaves and wilting or dying plants, to allow intervention prior to significant plant losses when required.


    Stand establishment steps to reduce flea beetle pressure

    Seeding date.Studies are inconclusive whether early or late seeding is best for flea beetle management. It may be region specific.Adults of crucifer and striped flea beetles overwinter in grasses and shelterbelts. Peak emergence of these overwintering adults occurs when soils reach 14-15°C. [1] One Alberta study from the late 1990s found that canola seeded earlier in the spring is more likely to reach true leaf stages before peak flea beetle pressure occurs. With true leaves present, canola plants are less reliant on cotelydons for photosynthate production, so they can withstand more feeding pressure. [2]

    A North Dakota study from 2002-03 found that peak emergence of adult flea beetles usually coincided with early seeded canola (early to mid May). This resulted in greater feeding injury to canola seeded early than to canola seeded in late May. [3]

    Another Alberta study from 2001-04 [4] found that by seeding as early as possible, which is recommended to maximize yield, canola in central and northern regions of Alberta was at greater risk of flea beetle damage than in southern regions. Specifically, canola seeded in late April in the south escaped flea beetle damage while at sites in central and northern Alberta, adult flea beetle numbers and damage levels were highest in the April seeded plots.

    In general, the conclusion is that the yield benefit of early seeding outweighs the flea beetle risk — but growers must use seed treatments to control flea beetles and apply foliar insecticide if defoliation exceeds threshold levels. Canola seeded early into cool soils will emerge slowly and often unevenly. With fewer plants emerging and with slower growth, flea beetle damage damage can surpass economic threshold levels more quickly, leading to losses in yields and returns if required applications are not made in a timely manner.

    Use seed treatment.Certified canola seed comes treated with insecticide, but growers may have a choice of low or high rate treatments. High rates provide more days of protection. Growers who follow these other management steps may get enough protection from the lower rates. High-rate products can provide an economic return in areas known to have higher flea beetle populations most years. Higher-rate treatments can also be beneficial for early seeded canola that may need a longer duration of protection to allow it  to emerge and grow beyond the 4-leaf stage, when growth rate can better outpace feeding damage. Seed treatments that provided the longest flea beetle protection usually provided the best seedling establishment, highest plant weight and highest seed yield. [8] In some years, seed treatment may not be enough. Monitor as seedlings emerge. Get ready to apply a foliar spray if 25% of cotyledon leaf area is damaged and flea beetles are still actively feeding, as damage can surpass the economic threshold of about 50% damage quickly.

    Another factor in the seed treatment decision is the dominant flea beetle species in the area. There are two species of flea beetles that damage canola: crucifer and striped. Even though they are about the same size, the two beetles are distinct in their feeding habits. The striped flea beetle feeds more voraciously than the crucifer flea beetle, and the striped flea beetle is more resistant to effects of seed treatment insecticides. [9] In regions of western Canada where the striped flea beetle is the dominant species, such as in the Parkland ecoregion and in the Peace River region of northern Alberta and British Columbia, it is recommended that growers use the higher-rate seed treatments.

    Seed at higher rates.Higher seeding rates provide a denser canola stand. Given that flea beetle populations are fixed based on the number of overwintering adults, spreading that population over more plants per unit area generally means less damage per seedling, on average. [2] With less damage per seedling, more plants are likely to survive flea beetle feeding. This prevents or limits delayed maturity that can result from heavy feeding, and it also reduces the need for in-crop foliar insecticide applications. Increased seeding rates may also reduce damage by other insect pests, including root maggots.

    Zero till.Flea beetle damage tends to be higher for canola seeded into tilled soil compared to untilled soil. [5] Flea beetles prefer warm, dry conditions, which are generally associated with conventional tillage. For some growers, zero tillage alone may be enough to keep flea beetle feeding below economic levels, reducing the reliance on high rate insecticide seed treatments for protection. [5] Growers in a zero till system will still need to assess this risk on their own farms and work with their seed supplier to choose a seed treatment that provides an appropriate level of protection.

    Wide row spacing.The same Alberta study [5] found that wider row spacing can also reduce flea beetle damage for the overall field, but the study’s recommended minimum spacing for B. napus was 14 cm (or around 6”). Most growers use row spacing of 23 cm (9”) or greater.

    Use large seed.An AAFC study found that canola seedlings grown from large seeds are more vigorous and may be more tolerant to flea beetle damage than seedlings from medium or small seeds. [6] When flea beetle damage exceeded 50% of the leaf area, seeds with thousand kernel weights of 3.6 grams and up had the best stand establishment, best shoot growth and highest yield.

    Cutworms

    Dingy cutworms overwinter as larvae and can start feeding early in the season. Redbacked, pale western and army cutworms overwinter as eggs, and their larvae can start feeding in early June. Seed treatments commonly used on canola do not provide protection from early season cutworm feeding, so growers need to scout for missing or cut plants and spray for cutworms if necessary.

    Growers breaking up forage crops for canola production should be prepared to spray for cutworms.

    Tillage in the spring can provide effective management in fields known to have high cutworm numbers. [7] This will set back the seeding date, but may protect the stand from severe cutworm damage. Allow volunteer growth to reach 3 to 5 cm (1.2 to 2”) then cultivate. This cuts off the food supply for larval cutworms. Follow with seeding 10 to 14 days later. Growers should weigh the potential for reduced cutworm damage against any negative effects tillage may have on the quality of the seedbed and its moisture status, which may reduce seedling survival. Reduced yield potential from delayed seeding is another consideration.

    While tillage may provide a short term benefit, zero tillage creates an environment favorable to long-term cutworm management. Research from Manitoba has shown that minimum tillage practices were associated with greater diversity of cutworms and their parasitoids, including some non-pest species of cutworms, than fields under conventional tillage. This increased diversity suggests a more stable ecosystem where cutworm outbreaks would be less common. [7]

    Wireworms and June beetle larvae

    Wireworms and June beetle larvae are becoming more common in fields across the Prairies, especially in fields recently converted from forages to annual crop production. These insects require monitoring from seedling emergence through to mid-rosette stages, but they rarely cause economic levels of damage in canola.

    Seed treatments are not registered to control or suppress wireworm or June beetle feeding on canola seedlings, but some treatments may provide limited protection. Due to the small size of canola seed and the low seeding rates of canola, seed treatments do not provide insecticide application rates sufficient to provide effective control of larger insects such as wireworms. Some early season suppression may occur  in some cases due to wireworm intoxication, but this would be a brief setback and not lead to wireworm death. No foliar-applied insecticides are registered to control wireworms or June beetle larvae in canola.


    References

    [1] “Emergence of overwintered and new generation adults of the crucifer flea beetle, Phyllotreta cruciferae (Goeze) (Coleoptera: Chrysomelidae)” B.J. Ulmerand L.M. Dosdall, Crop Protection 25 (2006) 23–30.

    [2] “Managing flea beetles in canola with seeding date, seeding rate and seed treatment.” Lloyd Dosdall and F. Craig Stevenson, Agronomy Journal, 2005.

    [3] “Impact of Planting Dates and Insecticide Strategies for Managing Crucifer Flea Beetles (Coleoptera: Chrysomelidae) in Spring-Planted Canola” Janet J. Knodel et al.  North Dakota State University, J. Econ. Entomol., 2008.

    [4] “Effects of seeding date and canola species on seedling damage by flea beetles in three ecoregions” Hector Carcamo et al, Journal of Applied Entomology, 2008.

    [5] “The effect of tillage regime, row spacing and seeding rate on feeding damage by flea beetles in canola in Alberta, Canada.” Lloyd Dosdall et al. Crop Protection, 1999.

    [6] “Effects of seed size and seed weight on seedling establishment, vigour and tolerance of Argentine canola (Brassica napus) to flea beetles, Phyllotreta spp.” R.H. Elliott et al, Canadian Journal of Plant Science, 2007.

    [7] Manitoba Agriculture, Food and Rural Initiatives factsheet: http://www.gov.mb.ca/agriculture/crops/insects/fad06s00.html

    [8] R.H. Elliott, Saskatoon Research Centre, Agriculture and Agri-Food Canada CARP Project # 2003-02-01-19.

    [9] "Differences inPhyllotreta cruciferaeandPhyllotreta striolata(Coleoptera: Chrysomelidae) responses to neonicotinoid seed treatments." Jim Tansey et al. Journal of Economic Entomology, 2008.