There are a number of insects considered to be pests of faba beans: grasshoppers, lygus bugs, blister beetle, aphids, and pea leaf weevil. Below is a description of these insect pests and control methods.
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.
No economic thresholds have been established for faba beans.
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. In the vegetative stage of the crop it is the defoliation by grasshoppers that are a concern. 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.
It is critical to apply only insecticides registered for use in faba beans and to respect the pre-harvest interval (PHI) of the insecticide selected to maintain crop marketability. All grasshoppers are susceptible to pyrethroids, certain organophosphate insecticides, and carbamate insecticides.
Lygus bugs can cause quality loss in faba beans, as they will move into the crop after other crops have matured and feed by using their sucking mouth parts to make pin holes in the seed coat.
Lygus bug infestations have posed a significant threat to profitable faba bean production in Alberta due to the extensive acres of canola. As canola is harvested first, the lygus bugs move into the still green faba bean fields and damage seeds in the developing pods. Human consumption faba beans have a very low tolerance for lygus bug damage (<1% for Grade No.1).
Caragana or Blister Beetle
Three species of caragana or blister beetle will attack faba beans. Blister beetles often attack faba beans in swarms, but generally in small patches within the field. They usually do not feed for very long before moving elsewhere. Therefore, control measures are difficult to employ and there are no established action thresholds for faba beans.
(Potato leafhoppers) are the vector insect for aster yellows of faba beans, but are also a sucking insect. Aster yellows is rarely yield limiting in faba beans. Faba bean damage from leafhoppers can consist of distorted growth and plant stunting. Populations mainly blow in from the southern United States, but there are also small native overwintering populations.
Aphids (Pea Aphids)
The pea aphid adult is small, about 4 mm (0.15 in) long, light green, and long-legged. The insect may be wingless or have prominent, translucent wings. Although pea aphids rarely survive winter in Saskatchewan, they may overwinter as an egg attached to the stems or leaves of alfalfa or clover. The eggs hatch in early spring and the young aphids feed on the newly emerged alfalfa or clover plants. During May and June, a new generation develops wings and with the aid of wind currents, fly to faba bean fields. The majority of aphids in faba bean fields are blown in on warm southerly winds from the United States in June or early July. The pea aphid weakens the plant by sucking its sap and is responsible for transmitting viral diseases. See Table 8 for insecticide control options.
Pea Leaf Weevil
Pea leaf weevil (Sitona lineatus) is a new and major threat in peas and faba bean. The main concern is larvae feeding on the nitrogen fixing nodules. Feeding results in characteristic leaf notches. Fortunately pea and faba bean seedlings can tolerate leaf notching and will usually recover unless there is very heavy feeding pressure.
The proportion of seedlings with terminal leaf damage (i.e. leaf notches) provides an adequate indication of overall plant damage, and to some extent, potential yield losses. It is therefore recommended to inspect the average feeding damage on the terminal first leaf in 10 seedlings, five along the perimeter of the field, and five within the field. The action threshold for control is 30 per cent damaged seedlings.
In the spring pea leaf weevil adults fly to legume fields where females lay eggs near the base of host plant. Larvae feed on rhizobium nodules. At maturity, larvae pupate in the soil and adults emerge to feed on leaves until late summer. Adults overwinter in field margins or in alfalfa. Fortunately, there is no evidence of Sitona lineatus overwintering in stored grain.
Pea leaf weevils are spreading outwards from the Southwest corner of Saskatchewan and are a pest to watch for.
Where yield loss occurs, is a primarily a result of the larvae feeding on the rhizobium nodules reducing nitrogen to the crop. Seed treatments are the only available control for larvae feeding in faba bean. Control products currently registered for pea leaf weevil control in faba beans include seed treatments from the insecticidal group neonicotinoids (Table 8).
Cutworms and wireworms can attack a wide variety of crop species and may attack faba beans. However, faba beans are very resilient as they have growing points near the seed that enables the plant to regrow following below ground insect feeding on the emerged or emerging seedling. Under high risk situations or where insect pressure is severe there are insecticides registered for use in faba beans.