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Field Peas – Insects

Generally, insects are not considered much of a problem in field pea crops in Alberta. Pea leaf weevils, grasshoppers and pea aphids are the main insects that could potentially cause economic damage. Blister beetles can occasionally be found in pea crops, but have not caused economic damage.

LINKS

Grasshoppers

Grasshopper, Photo Credit: Shelley Barkley

DAMAGE
  • Field peas are not considered a preferred food of grasshopper, so they will normally attack other crops before moving into a pea field – with direct seeding of pea into a perennial forage stand, however, grasshoppers can become a problem.
  • Grasshoppers can attack at any growth stage, but cause the most damage in the pea seedling stage – grasshopper nymphs will attack seedlings on the edge of a field as they emerge from ditches and fence rows in the spring.
  • In the bud stage through to early pod development they eat flower buds, open flowers, and developing pods.  Feeding on early developing pods can result in yield loss and cause delay in maturity as the plant tries to compensate for the lost biomass.

LIFE CYCLE
  • Overwinters as eggs in pods (8 – 150 eggs/pod) laid in soil and hatch the following spring when the temperature reaches 4.5°C.
  • One generation per year.

CONTROL
  • Early seeding can help pea plants outgrow the susceptible stage before grasshoppers emerge.
  • To monitor, start from a corner of the field, sample at least twenty sites along a line to the field centre, then to one side. Count the number of nymphs that jump in a 1 ft2 area as you approach each site (e.g. every 100 steps). Divide the total number of grasshoppers counted by 2 for number/m2.
  • Check field margins for grasshoppers moving in from roadsides and headland. Numbers will be higher in field margins and a thick crop will deter the insects from moving further into the field as they prefer more open and bare areas. Also check around wet areas in drought seasons.
  • No insecticides are registered for the control of grasshopper in pea.
  • Natural predators include birds, small rodents, coyotes, parasitic and predatory insects, as well as the pathogenic fungus Entomophthora grylli Fresenius, and the microsporidian parasite Nosema locustae Canning.

ECONOMIC THRESHOLD
  • Economic threshold in pea is 10 grasshoppers per square metre.

REFERENCE

Pea Aphids

Pea Aphid, Multiple Life Stages, Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

DAMAGE
  • The pea aphid is recognized as a yield-threatening pest in peas.
  • The pea aphid adult is small, about 4 mm (0.15 inches) long, light green, and long-legged. Adults can develop wings, but are often wingless.
  • The pea aphid weakens the plant directly by sucking its sap, and can also spread viruses from infected plants to healthy ones. In warmer countries, they are also responsible for transmitting virus diseases.
  • Aphids reproduce more slowly under cooler conditions, where pea is generally grown in Alberta, so rarely cause problems.
  • Damage caused by aphids can be difficult to quantify because they rob plants of resources by piercing and sucking rather than chewing and defoliating.
  • Populations can increase rapidly due to their ability to give birth to live young without mating, and because they can easily migrate into areas on air currents.
  • The population must reach threshold levels prior to the plant nearing maturity to cause damage.

LIFE CYCLE
  • Pea aphids overwinter as eggs on leaves and stems of perennial legumes such as the crowns of clover or alfalfa, but more commonly blow in on warm southerly winds from the United States in June and early July.
  • If the pea aphid arrives early enough and the environment is conducive for rapid reproduction, multiple generations of the insect may eventually result in numbers high enough to cause economic loss.
  • Up to 23 generations are produced asexually before winged females migrate to summer crop hosts where several generations are produced over the summer.
  • Colonies are generally less dense than other species attacking field crops.
  • Winged sexual forms are produced in late summer that mate and females return to winter hosts to lay eggs.

CONTROL
  • Scout for pea aphids at every flower. At four locations per field, check five plant tips (top 8 inches), or conduct 10 sweeps with a sweep net.
  • There are insecticides registered to control pea aphids (refer to The Blue Book for appropriate options). Insecticides with both contact and systemic action can be advantageous. 
  • Heavy winds and rain can minimize the damaging effects.
  • To avoid a recurrence of the problem after spraying, delay application of insecticide until late-flowering. One application per season should give satisfactory control.
  • Pea aphid populations usually begin to decline in mid-to-late August due to drying of the crop, parasitic wasps, diseases, and other factors.

ECONOMIC THRESHOLD
  • Economic threshold for spraying in pea if there are more than 10 aphids per plant in tenth node to early flowering stages. Population estimates should be calculated by averaging the counts taken from at least five separate areas of the field.
  • Economic thresholds are also based on when few natural predators are present and aphid numbers do not decline over a two-day period.
  • If the majority of the pea seeds are close to full size, the aphid is less likely to cause significant damage.

RESOURCE 

REFERENCE

Pea Leaf Weevil

Pea Leaf Weevil, Photo Credit: Shelley Barkley

Pea Leaf Weevil Emerging from a Root Nodule

Pea Leaf Weevil Damage on Field Pea

DAMAGE
  • Pea leaf weevil is a tiny insect that punches far above its weight in terms of potential impact on crop yield.
  • The size of a grain of rice, this non-native invasive insect has emerged in recent years as a threat to Alberta’s most-planted pulse crop.
  • Slender, greyish-brown, about 5 mm long with a short snout, with three light coloured stripes extending length-wise down the thorax onto the elytra. The mature larvae are 3.5-5.5 mm long, c-shaped, legless with brown head.
  • Complicating growers’ pea leaf weevil defense is the fact that this insect appears intermittently, and in some years it’s a significant problem, while in others it’s just a minor inconvenience.
  • The grey adult weevil feeds on leaf margins (notching) and growing points of host seedlings, but damage is non-economic.
  • Pea and faba bean seedlings can tolerate leaf notching, and will usually recover unless there is very heavy feeding pressure.
  • Usually pea plants will survive this defoliation, however, adult females will lay large numbers of eggs at the base of pea plants.
  • The main concern is when the larvae hatch and burrow into the soil.  They cause more serious damage by feeding on nitrogen fixing nodules on the roots of the plant, reducing nitrogen to the crop, resulting in poor plant growth and low seed yields.

LIFE CYCLE
  • The pea leaf weevil produces one generation per year.
  • The adult overwinters in soil within or adjacent to alfalfa, other perennial legumes, or tree shelters. In May through June, eggs are laid on or near developing pea plants.
  • In the spring adults disperse, up to a few kilometres, mainly by flying when temperatures are above 17°C or walking short distances. Each female lays up to 300 eggs throughout the summer in the soil near or on developing plants.
  • The larvae hatch in one to three weeks, and move to Rhizobium nodules on the root, where they feed.
  • Once the larval stage is complete, the insect pupates and emerges as an adult once again in late July through September.
  • Newly emerged adults search for any pulse crops to continue feeding before overwintering.

CONTROL
  • Occasional monitoring to look for the characteristic U-shaped notches on seedlings in early spring is advised.
  • Up to the 5th node stage, examine the clam leaf of 10 plants for the notches at each of five sites along the field edge and another five sites within the field when the pea crop is at the second or third node stage. If notches occur on the lower leaves but not on the clam leaf, then the weevil has likely already laid its eggs and it’s too late to spray.
  • The proportion of seedlings with terminal leaf damage (ex. leaf notches) provides an adequate indication of overall plant damage, and to some extent, potential yield losses.
  • Use seed treatments where pea leaf weevils are a constant threat.  Otherwise, apply recommended foliar sprays against adults as requiring.  Keep monitoring as weevils may re-invade fields.  

ECONOMIC THRESHOLD
  • The adult is difficult to observe; therefore, economic thresholds are determined by the severity of notches in the plants at various points of the field – 30% of seedlings with damage (leaf notching) on the clam leaf during the 2nd to 5th node stage.
  • The crop is not susceptible to damage after the 6th node stage or it is too late to attempt control.

RESOURCES

REFERENCE

Insects of less concern:

Blister Beetle

Nutttall’s Blister Beetle, Photo Credit: Shelley Barkley

DAMAGE
  • Adult blister beetles are 12-25 mm long; elytra are flexible and thorax is usually narrower than the round head and elytra.
  • Nuttall blister beetle adults are metallic green or purplish (16-28 mm long).
  • Adults are frequently found in groups feeding and mating on flowering crops but rarely cause damage before moving on. 
  • However, blister beetles contain the toxin cantharidin. It can cause severe distress in sensitive livestock, especially horses, after eating baled alfalfa hay containing blister beetles.
  • The larvae do not feed on crops.

LIFE CYCLE
  • Overwinter as larvae in the soils. Newly emerged adults congregate on food plants to feed and mate.
  • Females lay four or five batches of 200 – 400 eggs in the soil which take two to three weeks to hatch. 
  • Most adults are present from early June to mid-August depending on species. 
  • One generation per year.

CONTROL
  • Scouting not developed. Use a sweep net to collect adults congregating on plants.

ECONOMIC THRESHOLD
  • None established.

REFERENCE

Cutworms

New Pea Seedlings Showing Cutworm Damage

Cutworm

DAMAGE
  • Cutworms are the larvae of several different types of moths.
  • Cutworms can cause damage to newly emerged pea seedlings, and at times may require insecticide application.
  • The first generation of cutworm larvae is the most damaging. Most sever young plants from roots near the soil line as they feed at the base of the leaves; others feed on the roots and underground stems of cut plants. One larva can kill many plants until it pupates in the soil.
  • The feeding behavior will vary depending on species, but most feed at night and will be below the soil surface, up to a depth of 2 to 4 inches during the day.
  • Below-ground feeding cutworms (pale western and red-backed cut worms) cut plants off at-or-near the soil surface.
  • Foliar feeding cutworms (dingy and army cutworms) feed above-ground, consuming the plant foliage.
  • Pea crops can often recover from cutworm damage if cool, moist growing conditions occur. However, plants are set back and may not be competitive.

LIFE CYCLE
  • Cutworms are an early-season insect pest.
  • Moths move up from the southern U.S. each spring; females lay eggs on weed and grass hosts in fields or margins. 
  • Cutworms overwinter in the soil as pupae. The larvae emerge from the pupae in the spring and feed on new plants.
  • The mature larvae are up to 46 mm long, smooth and range from purplish to brown in colour. Patterned with grey lines and spots.
  • Adults are 20 mm long body with long, narrow, usually dark forewings which are pale near the tips with three black dashes on each forewing. Wing span varies from 38-50 mm.
  • There are one or two larval generations per year before populations die off in the fall.

CONTROL
  • Crops should be scouted regularly (weekly) in early spring to June.
  • Monitor germinating crop for expanding thinned or bare areas.
  • Look for areas with missing plants and wilted, notched, gouged or shot-holed plant tissue. Individual cutworms tend to feed within the seed row, leaving a gap when the affected plants are clipped off or die.
  • Scouting for cutworms is often more successful in the evening when they come out to feed. Cutworms are nocturnal and spend the day hiding under debris or in the soil.
  • Dig around in suspect areas with a trowel or a shovel. Sift soil or scoop soil into a container and shake loose any potential larvae.
  • Cutworms may be found anywhere from just below the soil surface to four inches deep. They move deeper into the soil during the heat of the day, so it may be necessary to dig deeper or revisit the field under cooler conditions.
  • Pheromone traps are available to detect emergence of adults. Monitor for expanding thinned or bare areas, especially sandy ridges and knolls. At a minimum 10 sites, mark off 0.1m2 area (1 ft2) and examine the top 2-3 cm of soil along the edges of the affected areas for larvae. Depending on the crop, other species of cutworms may be present.
  • Apply necessary foliar treatments in the evening just before larvae emerge from the soil to feed. Only infested areas need to be treated.
  • Natural predators include ground beetles that will attack exposed larvae on soil surface; eggs and larvae are attacked by insect parasitoids and predators.

ECONOMIC THRESHOLD
  • The economic threshold for cutworms of 2 – 3 larvae/m2 in top 7 cm of soil.

REFERENCE

Wireworms

Wireworm, Photo Credit: Shelley Barkley

Prairie Grain Click Beetle, Photo Credit: Shelley Barkley

DAMAGE
  • Wireworms are the immature stage or larvae of click beetles.
  • Wireworms are small-segmented, tan coloured worms that live in the soil and will feed on a variety of plant hosts, including field peas, dry beans, lentils, chickpeas, and soybeans. Although wireworms prefer grassy plants (cereal crops), wireworm damage has been noted in other crops including canola, chickpeas, and potatoes.
  • Wireworms tend to be more abundant in moist soils and in lower, damper areas of a field.
  • Wireworms feed on germinating seeds and on the underground parts of stems of young seedlings, causing reduced crop emergence and thinned stands. Damage is often blamed on poor quality or dry soil, and occurs only in the spring.
  • Damage can be seen in the field by wilting plants, resulting from wireworms chewing on the stems, and may appear similar to cutworm damage. The distinction is that wireworm feeding often looks more like the stem is shredded, and it is usually still attached to the roots, whereas cutworm feeding cuts the stem right off. Wireworms tend to shred the plant tissue below the soil surface.
  • Wireworms can eventually cause the death of the plant.
  • Damage may not be noticed early enough in the season to reseed.

LIFE CYCLE
  • Only one generation per year, however the larval stage can last four to 11 years depending on species.
  • Both new adults and larvae overwinter. Larvae pupate in earthen cells, and the new adults remain in the cell until the following spring.
  • Larvae will migrate near the germinating soil in early spring, and move back down deeper in the soil as soil temperatures climb, later in the summer.
  • The mature larvae have an elongated, cylindrical, wiry body, 10 – 20 mm long; last abdominal segment is flattened with a keyhole-shaped notch.
  • Adult wireworms are click beetles that do not do economic damage to crops. They will characteristically make an audible clicking sound when they try to right themselves when placed on their backs (hence the common name – click beetles).

CONTROL
  • Controlling wireworm requires the use of an insecticidal seed treatment.
  • Predators of wireworms include ground beetles, rove beetles, and larvae of stiletto flies.

ECONOMIC THRESHOLD
  • None established.

REFERENCE

Other Insect

Lygus Bug
  • Because of their wide plant range, the lygus bug can potentially can be found in all pulse crops.
  • Resource: Lygus Bug

Adult Lygus Bug, Photo Credit: Shelley Barkley

Fifth Instar Nymph, Photo Credit: Michael Dolinski

 

Special thanks to Saskatchewan Pulse Growers and Manitoba Pulse and Soybean Growers.