Invasive insects and other invasive pests can have significant and negative impacts on agroecosystems and increase the cost of crop production. For example, the pea leaf weevil and cabbage seedpod weevil invaded and established in the prairie region in the past 25 years. Both have affected yield and required insecticide application for management.
Managing invasive alien species, including insects, involves: Preparedness, Prevention, Detection, Response and Recovery.
Preparedness, Prevention, and Detection are important steps that can help to keep invasive species from becoming established. Everyone can help to prevent the invasion of insects by following guidelines to avoid the accidental movement or introduction of insects to Canada.
Similarly, everyone can help with early detection of invasive insects. In the Prairie Region, 12 important insect pests to watch out for are included on posters developed by the Canadian Plant Health Council. The poster is also available in French.
This is the last Insect of the Week post of 2024. Thank you for reading the Insect of the Week series this year!
The wheat stem sawfly, Cephus cinctus, is native to western Canada. Its hosts include species of native grasses as well as rye, barley, and winter wheat. It is most economically damaging to spring wheat and durum wheat in western Canada.
An adult wheat stem sawfly. Picture by Dylan Sjolie, AAFC-Saskatoon.
Adult wheat stem sawfly do not feed; they live for about 10 days and spend that time mating, dispersing to wheat or durum crops, and laying eggs. Larvae are the damaging stage of the wheat stem sawfly life cycle. Larvae feed on pith inside the wheat stems. Because the larvae feed inside the wheat stems, the damage they cause is not obvious or easy to detect.
By feeding inside the host plant stems, wheat stem sawfly can reduce the quality and quantity of the grain produced by their host. In the late summer, the larvae also cut the stems of their host plant when they prepare to overwinter. The cut stems are susceptible to lodging.
European corn borer, Ostrinia nubilalis, is a generalist pest of a variety of crops, including corn, potatoes, beans, sugar beets, tomatoes, quinoa, and millet. European corn borer is an occasional pest of crops in western Canada; as many of its hosts are grown in western Canada, it is important to monitor for European corn borer. In eastern Canada, European corn borer is an important pest and its populations are monitored by the Great Lakes and Maritimes Pest Monitoring Network and by the Réseau d’avertissements phytosanitaires in Quebec.
European corn borer larva and feeding damage on a plant stem. Picture by Jocelyn Smith, University of Guelph.
In western Canada, European corn borer has one generation per year. Adult female moths lay eggs in clusters on the underside of leaves in June, July and August. Larvae can typically be found feeding on leaves starting in July. Larval feeding continues until the larvae reach the fifth instar, which overwinters.
External plant symptoms of European corn borer feeding damage on corn. Picture by Jocelyn Smith, University of Guelph.
Early instar European corn borer larvae eat the leaves of their host plants. As the larvae grow, they tend to begin feeding on the midrib of leaves and on plant stems by tunneling into these plant structures. In corn, the growing larvae can mine into and feed on the corn tassels. Damage to the leaves and stems caused by European corn borer larvae can cause dieback, especially to young plants. Damage to the stems caused by tunneling larvae can weaken the plants, leaving them prone to breakage during storms or in windy conditions. Damage to the stems also interrupts nutrient cycling in infested plants, which can affect the quality and quantity of yield.
European corn borer damage to millet. Note the pile of frass (insect feces) in a pile below the affected millet plant. Picture by John Gavloski, Manitoba Agriculture.
Later instar larvae often begin feeding on the reproductive structures of the host plant: the pods, fruits, or ear shanks. Larval feeding to these structures affects the marketability of the crop; in many crops, including sweet corn and peppers, there is zero tolerance for European corn borer damage.
The Insect Community of Practice, a committee of the Canadian Plant Health Council has developed a harmonized monitoring protocol for European corn borer. This protocol can be used in any of the many crops that European corn borer can infest, not just corn.
Pieris rapae is known as the imported cabbageworm, the cabbage butterfly, and the cabbage white butterfly. Larvae are green and are covered in short, soft hairs, giving them a velvety appearance. The larvae are the damaging stage of the cabbage white butterfly life cycle. The larvae will eat the leaves and pods of canola and related field crops, but cabbage white butterflies are not considered to be an economic pest of canola.
A group of larvae of the imported cabbageworm (a.k.a. cabbage white butterfly) feeding inside a broccoli crown. Picture by Meghan Vankosky, AAFC-Saskatoon.
The larvae of cabbage white butterflies also consume the leaves of cruciferous weeds and vegetables, like broccoli, cabbage, and rutabaga. In vegetables, feeding damage to the leaves results in jagged holes.
A broccoli plant in a backyard garden with jagged holes in the leaves caused by cabbage white butterfly larvae. At least two larvae are also visible in the picture. Picture by Meghan Vankosky, AAFC-Saskatoon.
Larvae can also tunnel into the heads of vegetables, as pictured below. The combination of feeding damage to the heads and build-up of frass (fecal matter) can affect the marketability of vegetables infested with cabbage white butterfly larvae.
A broccoli crown with two holes in the head where larvae of the cabbage white butterfly tunneled into the center of the crown. The larvae pictured above were found feeding inside this broccoli crown. Picture by Meghan Vankosky, AAFC-Saskatoon.
Adult cabbage white butterflies do not damage crops as they feed on nectar. The white butterflies have black-tipped forewings with black spots and are often seen flying around canola and mustard fields and around gardens where cruciferous vegetables are grown. In 2023, cabbage white butterflies were especially numerous in southeastern Saskatchewan in August.
An adult cabbage white butterfly. Picture by David Cappaert, bugwood.org.
Canola flower midge overwinter inside cocoons in the soil and adults usually begin to emerge in late June and can be found until late August, so long as canola flowers are available. There are likely two generations per year, but emergence of adults also seems to be relatively unsynchronized and may not always occur in distinct peaks that align with discrete generations. Canola flower midge adults are not damaging to their host plants. The presence of adult canola flower midge can be detected using pheromone-baited traps.
A canola flower with canola flower midge larvae developing inside. Picture by Jon Williams, AAFC-Saskatoon.
Adult female canola flower midge lay their eggs on developing canola buds before they bloom. The larvae develop in groups inside the flower, resulting in a galled flower that does not produce a pod.
Galled flowers, resulting from larval feeding by canola flower midge on a canola raceme. Picture by Boyd Mori, University of Alberta.
Galled flowers can occur at any point along a canola raceme, with early emerging adults laying eggs on the first flowers to open. The galled flowers tend to remain ‘stuck’ on the raceme, even after the larvae have dropped to the soil to pupate. A monitoring protocol for canola flower midge, based on galled flowers can be used to estimate population densities in canola fields.
A canola raceme with galled flowers (circled), caused by canola flower midge in the field. Picture by Boyd Mori, University of Alberta.
Canola flower midge was described in the scientific literature in a paper published by Mori et al. in 2019, after its identify was confirmed in 2016. There is still a lot to learn about canola flower midge, including its potential to have economic impacts on canola yield.
Please read more about canola flower midge in previous Insect of the Week posts published in 2018 and 2021 or visit the Canola Council of Canada Canola Encyclopedia for more information about canola flower midge.
To the very best of our knowledge, swede midge are NOT currently present in western Canada.
Adult and larval swede midge on a growing point of a canola plant; note the characteristic purpling of the infested growing point. Picture by Jon Williams, AAFC-Saskatoon.
Every year, the Prairie Pest Monitoring Network coordinates a pheromone-based monitoring program for swede midge because of the high risk that swede midge poses to the canola industry in western Canada. Swede midge is also a threat to the vegetable industry in western Canada, as it can use broccoli, cabbage, cauliflower, and other crucifer vegetables as a host.
Adult swede midge do not damage canola or crucifer vegetables, but females lay eggs on the growing points of the plant. When eggs are laid on the florets of canola, some flowers on the raceme may develop normally, but the others become ‘fused’ together as a result of swede midge larval feeding.
Flower buds ‘fused’ together on a floret, the result of infestation by swede midge. Picture by Jon Williams, AAFC-Saskatoon.
Female swede midge can also lay eggs where new racemes or branches grow off the main stem of canola plants. In this situation, larval feeding stops the growth of the new raceme, leaving a stunted raceme with crumpled leaves that often turn purple.
Crumpled and purple-coloured new growth on a canola plant, as in the circled part of the picture, is a symptom of infestation by swede midge. Picture by Meghan Vankosky, AAFC-Saskatoon.
In vegetable crops like broccoli, cauliflower, and cabbage, larval feeding on the growing point of the plant prevents development of the harvestable heads. Very high yield losses have been observed in eastern Canada and the eastern United States in vegetable crops because of swede midge damage.
So far, swede midge is not an established pest in western Canada and we have not found it in pheromone traps in 10+ years of monitoring in Alberta, Saskatchewan, or Manitoba. But, swede midge is slowly moving farther and farther west in the United States. To protect the canola and vegetable industries in western Canada, it is very important to be vigilant and continue to monitor for swede midge.
If you find damage on canola or crucifer vegetable crops that looks like it could be swede midge damage, please report it. You can email meghan.vankosky@agr.gc.ca with pictures or questions.
The Lygus bug pest complex includes at least 5 species, including the tarnished plant bug (Lygus lineolaris), Lygus keltoni, and Lygus borealis. Lygus bugs have an upside down triangle on their backs and adults are about 5-6 mm long; their colour varies depending on the species and their stage of development. Lygus bugs have a fairly wide host range, but canola, soybean and alfalfa are prone to yield losses resulting from Lygus feeding damage.
An adult Lygus bug, with the characteristic triangle on the back. Picture by AAFC-Saskatoon. Lygus nymphs of different instars and an adult Lygus bug. Pictures by Hector Carcamo, AAFC-Lethbridge.
The nymphs and adults use piercing and sucking mouthparts to drink from their host plants. They prefer to feed on new growth and reproductive tissues, as these are more nutrient-rich than other plant structures. Feeding by Lygus bugs can result in bud-blasting, where developing buds or flowers do not continue to develop and drop from the plant. If Lygus bugs feed on developing seeds, the seeds become shriveled, reducing yield quality and quantity. Watch for circular, black scars on canola pods, as these are an indicator that Lygus bugs has been feeding on canola pods, and probably on the seeds inside the pods.
Top: A canola raceme with bud blasting symptoms typical of Lygus feeding injury. Bottom: Canola seeds damaged by Lygus feeding (left, middle) and healthy canola seeds (right). All pictures by R.A. Butts, AAFC-Lethbridge.
In addition to direct yield losses due to Lygus feeding damage, the wounds left by their mouthparts make plant tissues vulnerable to infection by pathogens. Because Lyugs bugs inject digestive enzymes into the plant to help break down plant tissues for consumption, they can also vector plant diseases.
Use a sweep net to scout for Lygus bugs in canola and alfalfa crops. The PPMN has a monitoring protocol available here.
In canola, scout as flowering is complete and pods are beginning to ripen. Take 10 sweeps at 15 locations in the field and estimate the number of lygus nymphs and adults per sweep. Recent research suggests that the economic threshold to avoid yield loss in canola is 2-3 Lygus bugs per sweep; check out the Canola Council of Canada, Manitoba Agriculture, and Alberta Agriculture and IrrigationLygus pages for more information about Lygus bugs and economic thresholds in canola.
In alfalfa, scout at the start of the bud stage by taking 5 sweeps in at least 15 locations per field and estimating the number of Lygus nymphs and adults per sweep. In seed alfalfa fields, the economic threshold is 8 Lygus per sweep in at least 40 sweeps.
For more information about Lygus bugs, visit previous Insect of the Week articles and find the Lygus page in Field Crop and Forage Pests and their Natural Enemies in Western Canada, available in English and in French. SaskPulse also recently published an overview of Lygus impacts on pulse crops written by Jennifer Bogdan.
Cabbage seedpod weevil, Ceutorhynchus obstrictus, is an invasive alien insect. Cabbage seedpod weevil is present in Alberta, Saskatchewan and Manitoba but it has not yet been detected in the Peace River region. Both the adult and larval stages of cabbage seedpod weevil feed on canola, brown mustard, and wild mustard. Feeding by the larvae generally has a greater impact on crop yields than feeding by the adults.
A dead adult cabbage seedpod weevil, posed for a picture by Jon Williams, AAFC-Saskatoon.
Cabbage seedpod weevil larval feeding results in direct yield loss because the developing larvae consume developing canola and brown mustard seeds inside the pods. Each larva can eat up to 5 seeds during its development. In addition to the direct yield loss caused by the larvae, pods infested by cabbage seedpod weevil are more likely to shatter during harvest and are prone to secondary infection by fungal pathogens.
A cabbage seedpod weevil larva inside a canola pod, where it developed by consuming canola seeds. Picture by AAFC.
Adult cabbage seedpod weevil will feed on a variety of brassica species, both crops and weeds, but does not use yellow mustard as a host plant. In spring, adult cabbage seedpod weevil can be found feeding in patches of flix weed, hoary cress, stinkweed, and volunteer canola. The adults then disperse into canola and brown mustard crops, where they eat flower buds and flowers. This feeding damage can result in bud-blasting, but does not typically impact crop yields. The new generation of cabbage seedpod weevil adults that emerge in late summer can also feed on pods before the crops are harvested.
Exit holes in canola pods that were chewed by cabbage seedpod weevil larvae. Larvae exit the pods to pupate in the soil once larval development is completed. Picture by AAFC.
Scout for adult cabbage seedpod weevil as they disperse into canola fields and prepare to lay eggs. When scouting, take ten 180° sweeps at ten locations in the field. Count the adult weevils after each set of 10 sweeps and calculate the average number of adult cabbage seedpod weevil per sweep. Carcamo et al. published new research about cabbage seedpod weevil in 2019, where they found that the economic threshold for cabbage seedpod weevil is 2.5-4 adult weevils per sweep.
For more information about cabbage seedpod weevil, visit previous Insect of the Week articles and find the cabbage seedpod weevil page in Field Crop and Forage Pests and their Natural Enemies in Western Canada, available in English and in French.
Wheat midge, Sitodiplosis mosellana, is an important pest of spring wheat, winter wheat, durum wheat and triticale. Spring rye can also experience some damage from wheat midge. Adult wheat midge do not damage their host plant, but do lay eggs that give rise to the damaging larval stage.
Wheat midge larvae feed on the outside of wheat kernels. Larval feeding results in shrunken, shriveled, and/or cracked kernels. Larval feeding can also cause kernel development to be aborted.
Feeding damage caused by larval wheat midge reduces grain yield and the quality of the harvested grains. Grains with wheat midge damage are typically downgraded at the grain elevator.
Wheat midge damage to the kernels is not easy to detect and can go unnoticed. Therefore, scouting for adult wheat midge is necessary to determine if foliar insecticides may be needed to prevent female wheat midge from laying eggs. Scout for adults in the evenings, daily during the susceptible plant stage. Calm evenings are best for scouting. Count the adults on 4-5 wheat heads at 5 locations in the crop to estimate midge per wheat head. Economic thresholds are:
During bertha armyworm outbreaks, canola and mustard crops typically experience the most damage and highest economic losses. However, bertha armyworm also eat alfalfa, peas, quinoa, flax, potatoes, and other crop and weed plants. Adult moths do not damage crops. Larvae consume green plant tissues using their chewing mouthparts.
A canola field stripped of leaves and with damage to pods following a severe infestation of bertha armyworm in Manitoba. Picture by John Gavloski, Manitoba Agriculture.
Bertha armyworm larvae are cause for concern when they occur in high numbers when canola pods are developing and maturing. This is because ‘mature’ bertha armyworm larvae (e.g., 5th and 6th instars) will start eating developing canola pods. Pod damage includes debarking, which can result in pod shatter before or during harvest. Bertha armyworm larvae can also directly consume the developing seeds. Bertha armyworm larvae can also clip flowers and bolls off of flax plants.
A bertha armyworm caterpillar in the process of feeding on a canola pod. Picture by John Gavloski, Manitoba Agriculture. Bertha armyworm damage, caused by larvae, to canola pods. Picture by Shelley Barkley, Alberta Agriculture and Irrigation.
The Prairie Pest Monitoring Network, Alberta Agriculture and Irrigation, Saskatchewan Ministry of Agriculture and Manitoba Agriculture coordinate an annual monitoring program for bertha armyworm using pheromone traps. The 2024 monitoring season started the week of June 10 and will continue until late July. Thank you to all of the volunteers across the prairies who are hosting bertha armyworm pheromone traps!
The number of bertha armyworm moths captured in the pheromone traps serves as an estimate of local risk. Watch for information about the bertha armyworm monitoring program from the PPMN Weekly Updates and the provincial insect updates. If trap catches indicate possible risk, then scout canola crops for larvae using the bertha armyworm monitoring protocol found on the PPMN Protocol page. Information to calculate economic thresholds can also be found in the monitoring protocol.
The life cycle of bertha armyworm: A) eggs, B) larval stage, C) pupal stage, and D) adult stage. The larval stage is the only stage that actively damages crops. All pictures by Jon Williams, AAFC-Saskatoon.
Diamondback moth larvae have voracious appetites for canola, mustard, flix weed, and vegetables including broccoli, brussels sprouts, cauliflower, and kale. They are specialists of plants in the family Brassicaceae (formerly Cruciferae).
When diamondback moth larvae first hatch, they are very small and tunnel inside the leaves to eat, resulting in damage that looks like shot-holes and leaf mines.
The life cycle of diamondback moth and damage characteristic of B) first and second instar larvae that tunnel and mine leaves, often leaving ‘shot-hole’ damage, and C) third and fourth instar larvae that can eat entire leaves, except for the leaf veins. All pictures taken by Jon Williams, AAFC-Saskatoon.
Third and fourth instar larvae are larger and can consume entire leaves, leaving just the leaf veins. Larvae will also eat the buds, flowers and developing pods. Later in the growing season, as canola matures, diamondback moth larvae can strip the pods of any remaining green tissues.
Damage caused to a canola plant in a colony of diamondback moth maintained at AAFC-Saskatoon. This is an extreme example of the capacity of diamondback moth larvae to strip all green material from the stems, leaving a skeletonized plant with a frosted appearance.The picture also shows how frass (fecal material) can accumulate on the remaining plant tissues. Picture by Meghan Vankosky, AAFC-Saskatoon.
In addition to feeding damage, frass (or fecal material) excreted by diamondback moth larvae can affect the marketability and the quality of crucifer vegetables like broccoli, cauliflower, cabbage and brussels sprouts by contaminating or staining the developing vegetable heads.
Remember that the diamondback moth can have multiple generations per year and that each generation takes about 18-20 days (but can be shorter or longer depending on temperature). With each generation, there is potential for the population density to grow and exceed economic thresholds. Scout for diamondback moth by examining plants for larvae and estimate the number of larvae per m2 to determine if the population is nearing or has exceeded the economic threshold.
In canola, the economic threshold for diamondback moth larvae is 100-150 larvae/m2 when canola plants are immature and flowering. The threshold is 200-300 larvae/m2 when canola plants are mature.
Also, refer to the diamondback moth pages within the “Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and management field guide” (2018) accessible as a free downloadable PDF in either English or French on our new Field Guides page.
Grasshoppers have mouthparts designed for chewing. Grasshopper nymphs and adults efficiently consume host plant foliage using their chewing mouthparts. During serious outbreaks, grasshoppers can strip all leafy material off the host plant stems and can also be found eating the leaves on shrubs and trees.
A sweetclover plant stripped of all leaves by grasshoppers. Picture taken by Meghan Vankosky, AAFC-Saskatoon in southern Saskatchewan in late summer 2023.
Feeding damage caused by different grasshopper stages on the leaves of cereal plants. Two-striped grasshoppers were common at this location, near Langham, Saskatchewan in July 2023. Picture by Meghan Vankosky, AAFC-Saskatoon.
The four primary pest grasshopper species differ slightly in terms of their preferred hosts. For example, the clearwinged grasshopper (Camnula pellucida) prefers cereals and some succulent grasses, but tends to avoid broad-leaf species. The two-striped grasshopper tends to show a preference for more ‘lush’ hosts, including broad-leaf species like alfalfa and various pulse crops, as well as cereals and grasses.
The migratory grasshopper has a wide host range and can clip pods and grain heads on maturing crops to find green tissues to eat.
Grasshopper feeding damage on wheat seedlings inside a demonstration cage. Picture by Meghan Vankosky, AAFC-Saskatoon.
Cereal leaf beetle larvae eat the upper surface of leaves, leaving behind ‘windows’ of missing foliage that can look like white or yellow stripes on the leaves. Wheat is the preferred host of adult and larval cereal leaf beetle, but this pest will also consume barley, oats, rye, millet, wild oats, and other grasses.
In this picture, we see evidence of larval cereal leaf beetle feeding where green material is stripped from the leaf surface. We also see a female Tetrastichus julis parasitoid in the process of parasitizing a cereal leaf beetle larva! Picture by Emily Lemke and Karen Shamash, AAFC-Lethbridge.
Feeding damage caused by adult cereal leaf beetle does not typically result in yield loss to the crop. Rather, feeding damage caused by the larvae, especially to the flag leaf, results in lost yield and reduced crop quality.
In western Canada researchers expected cereal leaf beetle to become a widespread and problematic pest. This prediction has thankfully not yet come true, probably mostly due to the efficacy of an introduced parasitoid, Tetrastichus julis. Learn more about the parasitoid in one of the 2023 Insect of the Week posts!
The pea leaf weevil is now present in Alberta, Saskatchewan, and Manitoba. Learn more about the biology of the pea leaf weevil here and about its distribution across western Canada here.
An adult pea leaf weevil, Sitona lineatus, feeding along the margin of a field pea plant, leaving behind a ‘u’-shaped feeding notch. Picture by Jon Williams, AAFC.
This insect causes damage to field pea and faba bean crops during the adult stage and during larval development. Adult pea leaf weevil eat the foliage of their host plants, leaving ‘u’-shaped notches along the edge of the leaves.
A field pea plant with pea leaf weevil feeding damage. Picture by Meghan Vankosky, AAFC.
Adult pea leaf weevil will also feed on a variety of other legume and pulse crops, including alfalfa, chickpea, and soybean. Most adult feeding on these hosts takes place in early spring before pea and faba seedlings emerge, and again after peas and fabas have been harvested in summer and early fall.
Faba bean plants with adult pea leaf weevil feeding damage. Picture by Meghan Vankosky, AAFC.
Pea leaf weevil larvae damage the nitrogen-fixing root nodules on field pea and faba bean plants.
A root nodule from a field pea plant with damage caused by pea leaf weevil larvae. Areas with damage appear a dark pink colour. Picture by Meghan Vankosky, AAFC.
Wireworms are the larval stage of click beetles from the family Elateridae. Click beetles, the adult stage, do not cause damage to crops.
A cereal field with patchy seedling emergence and stand establishment due to wireworm infestation. Picture by Dr. Haley Catton (AAFC, Lethbridge).
Similar to cutworms, bare patches in a field can be an early and obvious sign of wireworm infestation in the spring. Patchy crop emergence, as pictured above, results when wireworms consume germinating seeds or feed on the roots and stems of young seedlings, as pictured below.
Wireworm feeding damage on a faba bean seedling. Picture by Chris Baan.
Wireworms live in the soil, where it can take 4 or more years to complete larval development. In the soil, wireworms feed on germinating seeds and the roots of a wide variety of prairie field crops including cereals, pulses, oilseeds, and vegetables including potato and carrots. Damage to root vegetables can result in unmarketable produce.
Carrots with damage caused by wireworms. Picture by Dr. Haley Catton (AAFC, Lethbridge).
It is common to use baits to scout for wireworms. Baits, consisting of cut potato pieces or soaked mixtures of oatmeal and other seeds, should be buried 5-10 cm deep at 10 or more locations in unplanted fields in the spring to determine if wireworms are present before planting. Leave the baits for 2 weeks and then dig up the baits to look for wireworms. In fields with patchy seedling emergence, soil sampling can be used to look for wireworm larvae and to determine if the damage is being caused by wireworms or by another pest (like cutworms).
The striped flea beetle and crucifer flea beetle are two of the most important pests of canola (and other Brassicaceae) in western Canada, especially early in the growing season. Adult flea beetles spend the winter sheltered under leaf litter, generally along field margins. In spring, adults disperse into crop fields to eat, mate, and lay eggs. For more information about the biology of flea beetles, click here.
‘Shot hole’ feeding damage caused by flea beetles on the cotyledons and first true leaves of a canola seedling. Picture by Ruwandi Andrahennadi, AAFC-Saskatoon.
Flea beetle feeding damage has a characteristic ‘shot-hole’ appearance on the cotyledons, as pictured above. Flea beetle feeding damage can also be observed on the first true leaves (also with a ‘shot-hole’ appearance) and on the stem and growing point of the seedlings.
A striped flea beetle feeding on the stem of a leaf. Excessive feeding on the stems of young seedlings can cause stems to break or plants to wilt and if severe, could kill the seedlings. Picture by Ruwandi Andrahennadi, AAFC-Saskatoon.
To scout for flea beetles, examine seedlings for the characteristic ‘shot-hole’ feeding, starting at the field margin. Scout often, as flea beetles can move into fields quickly. The action threshold for applying foliar insecticides for flea beetle is met when 25% of the cotyledon area has been eaten. Visit the Canola Council of Canada Canola Encyclopedia for tools to help estimate defoliation by flea beetles.
Flea beetles can also cause damage later in the summer when the new generation of flea beetles emerges and are looking for food before winter. The feeding damage looks the same as the damage in the spring. High densities of flea beetles feeding on plants late in the season can cause plants to ripen prematurely and feeding damage on pods can contribute to yield loss via pod shatter.
Drying and desiccated leaves of rutabaga in late summer at Outlook, Saskatchewan, following a severe infestation of new generation flea beetles. Picture by Meghan Vankosky, AAFC-Saskatoon.
Welcome back to the Prairie Pest Monitoring Network Insect of the Week! In 2024, the Insect of the Week theme is “What is eating my crop?” All of the Insect of the Week posts will focus on the damage that insects pests cause to their host plants. In many cases, insect feeding damage is characteristic of certain pest species and can help to identify the insect pest or narrow down the list of suspects.
Cutworm Damage
Numerous species of cutworms can cause economic damage to crops in western Canada, including pale western cutworm (Agriotis orthogonia) and redbacked cutworm (Euxoa orchrogaster). Because there are so many species of cutworms, cutworms are an important pest complex, with quite diverse life histories, preferred host plants, and damage symptoms.
All cutworm species undergo complete metamorphosis during their lifetime, progressing through four stages: egg, larva, pupa, and adult. The larval stage is responsible for damage to crops and forage plants. Several cutworm species overwinter as larvae in western Canada, including army cutworm (Euxoa auxiliaris), dusky cutworm (Agrotis venerabilis), and glassy cutworm (Apamea devastator). Therefore, there might already be larval feeding activity happening where these species are present.
A cereal field with patchy crop establishment due to cutworm infestation (picture courtesy of AAFC)
There are three primary types of feeding behaviour used by larval cutworms:
1) Subterranean larval feeding, where larvae cut the main stem and consume the foliage by pulling it underground. These larvae are almost never seen out of the soil. The glassy cutworm is an example of a subterranean cutworm. Bare patches in crops, as pictured above, can be indicative of larval feeding by subterranean cutworms.
2) Defoliation by above-ground and surface-feeding larvae that feed on foliage at night but spend the day hiding under leaf litter or under the soil. The army cutworm is a typical above-ground feeding cutworm; late-instar larvae will eat entire leaves, while young larvae feed along the leaf margins. Damage typical of the black cutworm (Agrotis ipsilon) includes irregularly shaped holes in the leaves and stem cutting.
3) Defoliation by climbing cutworms, where the main stem is not usually damaged but is used by larvae to reach the leaves. Damage to the foliage is similar to that caused by above-ground or surface-feeding cutworms.
For more information about cutworms and the damage that they do, please check out Cutworm Pests of Crops on the Canadian Prairies, available in English and in French. You can also read about cutworms in Field Crop and Forage Pests and Their Natural Enemies in Western Canada (in English or in French).
