Insects and Pests of Field Crops: Canola Insects and Pests

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Pub 811: Agronomy Guide> Insects and Pests of Field Crops > Canola Insects and Pests

Excerpt from Agronomy Guide for Field Crops
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Table of Contents

• Introduction
  • Table 13-8, Canola Insect Symptoms in the Field
• Crucifer Flea Beetle
• Striped Flea Beetle
• Cabbage Seedpod Weevil
• Swede Midge
• Diamondback Moth
• Tarnished Plant Bug
  

Introduction

This section describes insects and pests that affect only canola crops. The insects and pests listed below affect canola as well as other crops.

Slugs
Tarnished Plant Bug

Table 13-8, Canola Insect Symptoms in the Field, shows insects and pests that could be causing the symptoms in the field.

Crucifer Flea Beetle (phyllotreta cruciferae)

Striped Flea Beetle (Phyllotreta striolata)

Description:

There are two species of flea beetles that attack canola. The striped flea beetle is approximately 1.5 mm (1/20 in.) long and has two cream-coloured stripes along its back. The crucifer flea beetle is bluish-black, 1.5 mm (1/20 in.) long and does not have stripes Plate 101. Flea beetles jump when disturbed, hence their name. The larvae are white, approximately 3 mm (1/8 in.) in length and have brownish heads.

Plate 101. Striped (left) and crucifer flea beetle adults. Flea beetle injury can be severe during the first three weeks after the emergence of spring canola.

Life History:

The adult beetle over-winters in sheltered areas such as woodlots under leaf litter. In early May, adults emerge and begin feeding on newly emerged canola seedlings. Hot, sunny weather promotes feeding damage. Eggs are laid on or in the soil between May and late June. Young larvae hatch and feed on the roots of the developing crop. Larval feeding occurs for approximately 1 month from June to late July. The larvae then pupate, and adults emerge in early August and feed on canola crops until late October. The adults then locate over-wintering sites. There is one generation per year.

Damage:

Both the adults and larvae feed on canola. However, it is the adult stage that is considered to cause damage. Damage is most severe during the initial 3 weeks following plant emergence. Spring adults feed on the leaves of young seedlings, causing a shot-hole appearance. Leaves and plants eventually wilt and die. Stands become thinned, and plants may become stunted. High infestations can cause up to 50% yield reduction. Once the crop reaches the three- to four-leaf stage, the crop can withstand the damage. Late-season adult feeding is usually not a concern (except in winter canola). Hot, dry weather favours this pest. Adults can enter spring canola fields during hot, dry conditions and feed on the surface of the pods, reducing seed development and increasing shatter.

Scouting Technique:

Scout newly emerged canola fields every 2 days, especially along border rows, for the migration of over-wintering adults from the fencerows and woodlots. Assess 10 plants in five locations across the field for feeding damage. Determine the average percentage of defoliation occurring. Monitor fields closely for pinhole feeding damage until the plants are past the 4-leaf stage.

Action Threshold:

Canola seedlings can withstand up to 25% defoliation in the cotyledon stage under good growing conditions without a significant reduction in yield. Once the crop reaches the 3-4-leaf stage, the plants are generally established and can compensate for the feeding damage. No thresholds are available for late-season pod feeding by the second generation of adults. Only when adult populations are extreme, and dry conditions are hindering the crop from compensating for the pod damage, is a spray recommended.

Management Strategies:

Seed treatments containing an insecticide are necessary at planting time to control flea beetles because of the difficulty in predicting their populations. A foliar application of insecticide may still be required should adult activity continue and reach threshold after the seed treatments are no longer effective. Seed treatment insecticides differ in the length of control of flea beetles, so some products should be used in combination with a granular insecticide, which provides longer protection. If damage is isolated along border edges, apply spot treatments. Canola is a valuable foraging crop for honeybees. Avoid applying insecticides during bloom. Removing cruciferous weeds such as mustards along field boundaries will remove alternative food sources.

Cabbage Seedpod Weevil (Ceutorhynchus obstrictus)

Description:

The adult cabbage seedpod weevil (CSW) is ash-grey to black in colour and approximately 4 mm (1/6 in.) in length. Like all weevils, it has a snout that resembles an elephant's trunk Plate 102. The larva are white, C-shaped and legless and can be found only within the pod.

Plate 102. Cabbage seedpod weevil is ash-grey to black and has a snout that resembles an elephant's trunk.

Life Cycle:

There are two generations per year. In the spring, adults emerge from their over-wintering sites including shelterbelts, leaf litter, fencerows and ditch banks. These newly emerged adults feed on the canola crop and other host plants, including volunteer canola and mustard plants. After mating, the female lays her eggs, typically one per pod, directly into the seedpod itself. The larvae then hatch within approximately 1 week, depending on temperature, and can consume 3-5 seeds during its development. Once mature, the larvae mine out of the pod, drop to the ground and pupate in the soil. Adults emerge from the soil 10 days later to feed on cruciferous plants until it is time to enter their over-wintering sites. The entire life cycle of the insect takes approximately 6-8 weeks. Host plants include the Brassicaceae (mustard) family (i.e., canola, broccoli and cauliflower) and cruciferous weeds (e.g., wild mustard, flixweed and stinkweed).

Damage:

Cabbage seedpod weevil is a serious pest in winter canola but can also impact early-planted spring canola. Over-wintering adults enter the canola near flowering and may feed on the flower buds, resulting in blasting. Summer-emerging adults can also cause injury by feeding directly on the green pods of later-planted fields. Pod feeding by the larvae can cause up to 35% yield loss. Yield loss is mainly due to the larval feeding injury, either directly from seed feeding or indirectly from premature pod shattering or seed exposure to diseases via the exit holes. Brown seed incidence has also been linked to injury by this pest. Spring canola fields tend to be less attractive, unless they are the only canola field in the area for adults to go to. High-risk fields include the earliest emerging winter canola fields in an area, as well as fields in a spring following a mild winter.

Scouting Technique:

Focus scouting on monitoring the adult population. Use a sweep net for sampling to determine population numbers. Begin sweeping when the crop enters the bud stage until after flowering. Take 10 sweeps (in one 180° arc) in 10 locations of the field and determine the average number of adult weevils per sweep.

Action Threshold:

No action threshold has been established for Ontario. Winter canola is at such high risk of injury from this pest that control through foliar insecticides is recommended, as long as adults are active in the field. In spring canola, control is not necessary unless at least 2-4 weevils per sweep are found in very early planted fields.

Management Strategies:

Research conducted in Ontario by the University of Guelph and OMAFRA indicates the importance of managing CSW in winter canola during early flowering. If budget only allows for 1 application of insecticide, the optimum timing is at mid-flowering, 7-10 days after the first flowers are noticed in the field. If budget allows, the greatest yield protection occurs when two applications of insecticide are made, one at first-flower and a second application 7-10 days later, during mid-flowering. Research did not show a value to spraying in the spring canola crop unless the field was planted very early and thresholds have been reached. Ensure that adults are actively feeding in the field prior to spraying. Spray in the very late evening or early morning when bees are less likely to be foraging in the crop. Contact local beekeepers at least 24 hours in advance so they can move their hives prior to spraying. A parasitic wasp has been found to help control this pest, though insecticide applications will be detrimental to this wasp. Control cruciferous weeds (i.e., mustard, stinkweed) and volunteer canola plants that can act as hosts.

Swede Midge (Contarinia nasturtii)

Description:

The adult swede midge is a very tiny light brown fly roughly 1.5-2 mm (1/16 in.). It is difficult to properly identify from other closely related midges. Larvae are small (0.3-3 mm when mature), off-white-to-yellow maggots that congregate at the growing point within the plant.

Life Cycle:

This pest has been confirmed to be present throughout Ontario and is now being detected in other states and provinces. There are four overlapping generations per year in Ontario starting in mid-May until October. Swede midge over-winters as a pre-pupa in the soil, and most adults emerge from mid-May to early June. Once mated, the female will lay her eggs on a host crop, typically in clusters of 2-50 eggs on the youngest and most actively growing portions of the plant. Once mature, the larvae fall to the soil to pupate. The pupa can survive in the soil for up to 2 years before emerging as adults. Rain triggers the pupae out of dormancy. Adults are weak fliers and are carried by wind. Pupae may be carried in soil on farm equipment.

Damage:

Only spring canola is at risk of injury from this pest, as infestations in winter canola in the fall do not impact yield. Impact in spring canola is strongly dependent on planting date (i.e., crop stage at time of infestation), crop rotation and infestation levels. Late-planted spring canola fields are most at risk. If the plants are infested at a very young crop stage, the canola plants can be extremely stunted and malformed. Enzymes in the saliva of the larvae break down plant tissue. On young plants that have not bolted yet, the larvae feed on the growing point, causing it to deform Plates 103, 104, 105, on page 296). The plant becomes stunted, struggles to bolt and produces very few, if any racemes. Without racemes, flower and pod production is impeded. If the canola plant is beyond the bolting stage before the midge enters the plant, the impact is usually not as extreme. Some racemes may become deformed and some pods develop into one cluster or bouquet but still produce seeds and therefore yield. Winter canola fields avoid much of the injury in the spring and summer because they are much more advanced in growth stage than spring canola.

Plate 103. Cabbage seedpod weevil larvae hatch inside the canola pod and feed directly on the seed, each consuming three to five seeds.

Plate 104. Swede midge damage to a young plant prior to stem elongation can result in no racemes, flowers or pods being produced.

Plate 105. Swede midge injury to plants already bolted results in all pods forming from injury site, causing a bouquet of pods. Yield is not impacted in this case.

Scouting Technique:

Adults can be monitored through the use of pheromone traps. This indicates when adults are flying around and laying eggs. On injured plants, split open the growing point where the malformation of the tissue is located, typically in unexpanded clusters of flower buds. Look for small maggots feeding. This will help confirm that injury is caused by swede midge and not due to herbicide injury application or nutrient deficiency issues.

Action Threshold:

No thresholds are available at this time. Control is mainly done through cultural practices.

Management Strategies:

Crop rotation and planting date are the most effective methods of control. Plant spring canola fields no later than the first week of June. If fields cannot be planted prior to this date, consider planting a different crop that is not a host to this pest. In fields of known infestations, rotating out of canola and other cruicifer crops for at least 2 or more years, including neighbouring fields within 1 km, will help reduce populations since swede midge are such poor fliers. If these cultural methods cannot be applied to control this pest in fields with a history of damage, apply a foliar insecticide application during bud-stage, just prior to stem elongation to protect the plant from injury so that bolting can still occur. Control all cruciferous weeds that can act as alternative hosts, including mustard, stinkweed and volunteer canola. Clean all farming equipment that is used in infested fields. Leave infested fields until last when working in fields to reduce the risk of spreading the insect to non-infested fields.

Diamondback Moth (Plutella xylostella)

Description:

The adult is a small, light greyish-brown moth approximately 1 cm (1/2 in.) long Plate 106. It is best identified by the white, diamond-shaped markings along its back when the wings are at rest. The larvae are light green-to-yellowish-green with a brown head and are approximately 8 mm (1/3 in.) long Plate 107. Larvae will wiggle backward when disturbed or drop from the leaves on a silken thread.

Plate 106. Diamondback moth adult is best identified by the white, diamond-shaped markings along its back when the wings are at rest.

Plate 107. Diamondback moth larvae are light green to yellowish-green, with a brown head. Larvae will wiggle backwards when disturbed.

Life History:

Diamondback moths do not over-winter in Ontario but are carried in on southerly weather fronts in early May. There are three to four generations per year in Ontario. The adult moth typically lays her eggs on cruciferous weeds such as mustard as well as volunteer canola. Larvae hatch within a few days and create mines or tunnels in the leaf tissue. As the larvae develop, they begin to feed on the exterior leaf tissue. Larvae then pupate on the plant in cocoons and adult moths emerge within 2 weeks.

Damage:

The main damage is caused by second-generation larvae, when plants are flowering and pod development begins. Young larvae feed on internal leaf tissue on the undersides of the leaf by creating small mines. More mature larvae, when numerous, will feed on the flowers and on the surface of young pods. Damaged pods do not fill correctly. Severe infestations cause the crop to appear white.

Scouting Technique:

Examine 10 plants in five locations across the field twice weekly during the growing season. Gently pull out shot-holed plants and shake them on a piece of paper and count the number of larvae present.

Action Threshold:

At podding stage of canola, 200-300 larvae/m2 (approximately two to three larvae per plant). If infestation in the spring occurs very early in a late-planted canola field and the larvae feed on the developing buds, control may be necessary. At the early flowering stage, the action threshold is lower, and control is recommended when populations exceed 100-150/m² (one to two larvae per plant).

Management Strategies:

• Rescue treatments are available when populations reach threshold.
• Canola is a valuable foraging crop for honeybees. Avoid applying insecticides during bloom.
• The adult diamondback moth population can be monitored using pheromone traps to warn of a possible infestation.
• Eliminate volunteer canola and cruciferous weeds that act as the initial food source for the spring generation.
• Cool, wet weather reduces moth flight activity, and adult females often die before laying their eggs.
• Humid weather following a rain helps promote the spread of a fungus that kills the larvae. Several predators and parasitic wasps also help control this pest.
  

Tarnished Plant Bug (Lygus lineolaris)

Tarnished plant bug (TPB) is an important pest of canola For information on this insect,.