Grasshoppers

DAMAGE

• Grasshoppers can be a major pest of faba beans. There are more than 80 species of grasshoppers on the prairies but only three species threaten faba beans. These three pest grasshoppers generally favour faba bean foliage and will feed on faba beans even when other food sources are available.

• The three species of grasshopper that threaten faba beans are: 

  • Melanoplus bivittatus (Two-striped grasshopper)

  • Melanoplus packardii (Packard grasshopper), and 

  • Melanoplus sanguinipes (Migratory grasshopper).

• They are all spur-throated and have a spine below the head. The first adult grasshoppers that appear on the prairies by late May are not typically pests. 

• In the vegetative stage of the crop it is the defoliation by grasshoppers that are a concern.

• Grasshoppers pose the greatest threat from the bud stage through to early pod development as they eat flower buds, open flowers, and developing pods. In this case, yields can be reduced by as much as 90%.

• As the plants develop to reproductive stages, the buds and pods can be affected by grasshoppers and infestations can effect pod formation, seed development, and ultimately yield.

• 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

• Overwinter 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

• 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 lentil 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.
• If grasshopper populations only exceed the economic threshold in the field margins, an edge treatment with an appropriate insecticide can save time and reduce costs, while providing adequate control.
• If grasshopper control is required based on numbers sufficient to cause economic damage, the optimal insecticide timing is when nymphs are at the third instar stage, which is usually about mid-June.
• It is critical to apply only insecticides registered for use in faba beans and to respect the pre-harvest interval of the insecticide selected to maintain crop marketability.
• All grasshoppers are susceptible to pyrethroids, certain organophosphate insecticides, and carbamate insecticides.
• 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

• No economic thresholds have been established for faba beans.

REFERENCE

• Field Crop and Forage Pests and their Natural Enemies in Western Canada:  Identification and Management, Agriculture and Agri-Food Canada, page 105 – 106.

Pea Leaf Weevil

Pea Leaf Weevil Emerging from a Root Nodule

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 field pea, faba bean, and chickpea pulse crops.
• The pea leaf weevil is slender, greyish-brown, about 5 mm long with a short snout. Characteristic 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 faba 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, where 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 legume crops, or tree shelters. In May through June, eggs are laid on-or-near developing chickpea 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

• New Video: When Pea Leaf Weevil Attacks!
• Pea Leaf Weevil 2019 Map
• Expansion of Pea Leaf Weevil 2011-2019
• Pea Leaf Weevil
• Alberta Pulse Growers is funding several research projects (Carcamo, Evenden) related to preventing pea leaf weevil damage to crops.

reference

• Field Crop and Forage Pests and their Natural Enemies in Western Canada:  Identification and Management, Agriculture and Agri-Food Canada, p. 30.

Pea Aphid

DAMAGE

• The pea aphid adult is small, about 4 mm (0.15 inches) long, light green, and long-legged.
• 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.
• The insect may be wingless or have prominent, translucent wings.
• The eggs hatch in early spring and the young aphids feed on the newly emerged alfalfa or clover plants.
• 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.
• During May and June, a new generation develops wings and with the aid of wind currents, fly to faba bean fields.
• 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.
• Insecticides are registered to control aphids on peas and lentils only.
• Heavy winds and rain can minimize the damaging effects.
• 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

• No action thresholds for faba beans.

RESOURCE

• Pea Aphid

REFERENCE

• Field Crop and Forage Pests and their Natural Enemies in Western Canada:  Identification and Management, Agriculture and Agri-Food Canada, p.30.