Insects and Pests of Field Crops: Forage Insects and Pests
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Insects and Pests of Field Crops > Forage Insects and Pests
Table of Contents
This section describes insects and pests that affect only forages. The insects and pests listed below affect forages as well as other crops.
Table 13-4, Forage Insect Symptoms in the Field shows insects and pests that could be causing symptoms in the field.
Description: The adult is a flightless, dark-grey weevil approximately 12 mm (1/2 in.) in length Plate 89 . Larvae are small, white and legless with a light-reddish-brown head and can be found in the soil, feeding on or in the roots Plate 90).
Life History: The snout beetle has a 2-year cycle. Adults
emerge from their over-wintering sites in April, feed on new alfalfa shoots
and migrate into new fields to lay eggs. Adults may walk short distances
or may be carried longer distances via the transportation of soil, gravel,
hay, farm machinery and waterways. All the adults are female. The eggs
soon hatch and begin feeding on the main and side roots of the host plant.
Damage: This potentially serious pest of forage crops
was first discovered in Eastern Ontario near Prescott and Brockville.
Additional infestations have since been found in Kemptville and Ottawa.
The larvae girdle the taproot or gouge its surface, leaving deep spiral
grooves. Severely injured plants may appear yellow and leafless in the
fall. Adults feed on leaves and stems, causing only marginal damage. Damage
is most evident in late summer and early fall.
Scouting Technique: Scout early for signs of beetle
migration in infested counties of Eastern Ontario. Later in the season,
dig up wilted plants and check them for damage and the presence of larvae.
Alfalfa snout beetle has a wide range of hosts. Although it finds alfalfa
to be the most attractive crop, larvae of the insect may attack all species
of clover, grape and strawberry. They sometimes even feed on weeds, especially
ones with fleshy roots such as wild carrot and dandelion.
Action Thresholds: None available.
Description: The adult is a very small, black, hump-backed
fly. The larvae are small, pale yellow maggots found within tunnels in
the leaf tissue.
Damage:This pest of alfalfa is now a more serious problem
in Northern Ontario. Small pinhole punctures are left in the leaves when
the adult feeds and lays its eggs. The developing maggots feed inside
the leaflet, creating tunnels or mines between the top and bottom layers
of the leaf. These tunnels usually begin at the base of the leaflet and
widen towards the leaf apex, creating a "blotch" appearance
Plate 91). Feeding damage primarily decreases forage
quality and seldom causes yield loss.
Scouting Technique: Scout fields weekly to determine
the percentage of leaves with pinhole feeding.
Action Thresholds: Control is only necessary if more
than 40% of leaflets show adult pinhole feeding.
Description: The adult weevil is a brown snout beetle,
about 5 mm (1/5 in.) long, with a dark brown stripe extending from the
head down the centre of the back Plate 92.
Larvae are bright green with a black head, six legs and a distinctive
white stripe down the centre of the back. At full size, they are about
8 mm (1/3 in.) long (Plate 93).
Do not confuse the alfalfa weevil larvae with the clover leaf weevil larvae, which are larger, have a light brown head, not a black head, and a white stripe edged with pink. Also, do not confuse alfalfa weevil larvae with predacious fly larvae, which have no head capsule and are legless.
Life History: Adults over-winter in plant debris and emerge in spring to feed on new alfalfa growth and lay their eggs in alfalfa stems in May. Larvae hatch from eggs and crawl to the tops of alfalfa where they feed on the developing leaf and flower buds. After feeding, larvae form loosely woven white cocoons in leaf masses and enter the pupa stage, usually in late June or early July. Pupae hatch in 1-2 weeks into the adult stage.
Damage: The larvae cause most of the damage as they feed within the leaf buds and then move to the tips of the plant. Damage starts out as pinholes and progress to feeding between the leaf veins, resulting in a skeletonized appearance. In heavy infestations, larvae shred the leaves so badly that fields take on a greyish-white or frosted appearance. Adult feeding throughout the summer does not cause significant damage.
Scouting Technique: Examine each field twice a week from mid-May to June. Check several areas throughout the field. Look for damage to show up first on shallow soils or on southerly slopes, particularly during warm, dry springs. Experience in Ontario has shown that the peak of larval attack usually coincides with the bud stage of the first crop. To count larvae, collect 30 stems in an M-shaped pattern. Place them inside a white pail and beat them against the side to knock off the third-to-fourth-stage instar larvae. First and second instars are smaller (3 mm or less), pale yellow-to-light green, with the white stripe not yet distinguishable. They may be in the upper leaves but do not include these younger larvae in the count. Check to see whether the weevil larvae look active and healthy. Larvae infected by the fungus pathogen are slow-moving, yellow or tan.
Occasionally, if weevil populations are high on a late first cut, surviving
larvae will feed on the re-growth. Such feeding can eliminate alfalfa
re-growth, which may lead to a loss of the stand. With a severe infestation,
be sure to monitor stubble re-growth. The characteristic symptom is the
alfalfa plant not "greening up," due to weevils feeding on the
developing crown buds. The presence of two or more active larvae per crown,
or 4-8 larvae per square foot (30 cm2) indicates a need to spray the stubble
True armyworm is an important pest of forages. For
information on this insect.
alfalfa, dry edible beans
Description: The potato leafhopper (PLH) adult is a
pale green, wedge-shaped, winged insect about 3 mm long with piercing
and sucking mouthparts Plate 94. It is most broad
towards the head, tapering evenly to the wing tips. It has a row of six
rounded, white spots behind the head. Nymphs are smaller than adults and
Life History: PLH do not over-winter in Ontario but
migrate north every spring, carried by weather fronts that start in the
Gulf of Mexico. Adults may arrive in late spring and begin sucking on
plant juices. Females lay their eggs in the tissue of main veins and petioles
of leaves. Development from egg to adult takes approximately 4 weeks.
Damage to Alfalfa: Damage is most severe in new seedlings and young regrowth. While potato leafhopper nymphs and adults suck juices from plant foliage, they inject a protein that blocks veins. This causes the edges to become yellow and puckered, with a characteristic yellow "V" shape beginning at the tip of the leaves. When severe, the leaves appear burned, which is called "hopperburn" Plate 95. PLH feeding causes reduced stem elongation, reduced root development, leaf cupping and stunting. Yields can be lowered by as much as 50% with a severe infestation, accompanied by a reduction in protein levels of 2%-3%. Decreased stand vigour results in slow regrowth following cutting and increased winterkill. Border areas are usually affected first. The symptoms of potato leafhopper are commonly confused with herbicide injury problems and nutrient deficiency. Most of the damage occurs from June to mid-August. High-risk factors include hot, drier-than-normal seasons. Symptoms are sometimes confused with nutrient deficiency or herbicide injury, and are often dismissed as "drought damage."
Damage to Dry Edible Beans: PLH feed by piercing plant
tissue and sucking plant sap. This causes the leaves to curl and pucker,
and eventually the leaf edges begin to scorch. These symptoms are called
hopperburn. Border rows are affected first. Because yield is lost before
hopperburn is evident, do not use the presence of hopperburn as a management
guide. Leafhoppers tend to come into soybean and edible bean fields after
neighbouring alfalfa fields are cut. The symptoms of potato leafhopper
are commonly confused with herbicide injury problems, nutrient deficiency
and moisture stress during dry conditions. High risk factors include hot,
Scouting Technique for Alfalfa: See Using
Sweep Nets, for a discussion on the use of sweep nets for scouting.
Economic losses occur before plant symptoms develop, so it is important
to identify the presence of large leafhopper populations before the damage
occurs. It is particularly important to monitor new seedings.
2 Adults and nymphs.
3 The taller the alfalfa, the more leafhoppers can be tolerated before control is necessary.
Scouting with a sweep net will help you determine whether early harvest
or spraying is needed. Scout at intervals of 5-7 days, beginning after
first cut. Take 20 sweeps from five areas of the field beginning in late
June. Avoid field edges. Determine the average number of PLHs per sweep.
Next, take 20 alfalfa stems at random and record the average plant height.
Scouting Techniques for Dry Edible Beans: Walk in an "X" pattern. In 10 areas of the field, pick 10 trifoliate leaves that are newly and fully expanded from the centre of the plant canopy. It is important to note that PLH adults readily fly away when disturbed, which makes them difficult to count on excised leaves. Apply a foliar insecticide if a field is near threshold based on counting nymphs, and a number of adult insects are observed in the crop canopy.
Management Strategies for Alfalfa:
Management Strategies for Dry Edible Beans:
Description: European skipper is a sporadic pest of
timothy, both in hay and seed production. Larvae can usually be found
within rolled leaves where they feed. Younger larvae have black heads
that eventually turn brown. Mature larvae are light green, approximately
2.5 cm (1/10 in.)in length and have brown heads with two light bands.
The adult is a pumpkin-orange butterfly with a 2.5-cm wing spread that
skips about hay fields in midsummer.
Life History: Eggs over-winter on the stems of crop
debris and weeds and hatch in the spring. Young larvae roll themselves
up in the leaves and seal the leaves closed with silk webbing. Larvae
feed on timothy until late-June. The larvae then attach themselves to
grass stems or the underside of weed leaves and develop into chrysalids
(the pupa stage of the butterfly). In approximately 2 weeks, the adult
skipper emerges. There is one generation per year.
Damage: Larval feeding causes leaf margins to become
irregularly notched and when abundant can cause defoliation. When the
population is very high, the larvae will also feed on the heads of plants,
leaving only the stems remaining in a field. Adult skippers feed on the
nectar of flowers and weeds and do not cause any damage to plants.
Scouting Technique: Begin scouting for larvae by late-April. Remove five random, one-square-foot (30 cm2) samples of forage down to ground level and place them along with the old crop residue into a bag. Tie the bag and leave overnight at room temperature. The caterpillars will crawl out of the residue and can be counted easily.
Action Threshold: Six larvae in a one-square-foot (30
cm2) area found in the early, brown-headed stage.
Management Strategies: See OMAFRA
Publication 812, Field Crop Protection Guide, for insecticide recommendations.
For more information:
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