Insects and Pests of Field Crops: Forage Insects and Pests

 

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

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

 

Table of Contents

Introduction

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

European chafer
Fall armyworm
June beetle
Slugs
True armyworm

 

Table 13-4, Forage Insect Symptoms in the Field shows insects and pests that could be causing symptoms in the field.

Table 13-4. Forage Insect Symptoms in the Field
  Possible Insect Source
Symptom Grubs (European Chafer or June Beetle) Alfalfa Blotch Leafminer Alfalfa Snout Beetle Alfalfa Weevil True Armyworm Potato Leafhopper European
Skipper
Plants wilting or dying
Plants dying back or dead, particularly in sandy knolls and first-year fields
x
 
 
 
 
 
 
Deep spiral grooves in taproot
 
 
x
 
 
 
 
Feeding injury
Mines or tunnels between top and bottom layers of leaf
 
x
 
 
 
 
 
Pinholes or skeletonization of leaves
 
 
 
x
 
 
 
Leaf-margin feeding, feeding damage to blossoms
 
 
 
 
x
 
x
Leaf discolouration
V-shaped yellowing at leaf tip; leaves appear burned  
 
 
 
 
x
 

 

Alfalfa Snout Beetle (Otiorhynchus ligustici)

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).

Plate 89. Alfalfa snout beetle adults are approximately 12 mm (1/2 in.) long and cannot fly.

Plate 89. Alfalfa snout beetle adults are approximately 12 mm (1/2 in.) long and cannot fly.

 

Plate 90. Alfalfa snout beetle larvae girdle the taproot or gouge its surface, leaving deep spiral grooves. Larvae are white and legless and have a reddish-brown head.

Plate 90. Alfalfa snout beetle larvae girdle the taproot or gouge its surface, leaving deep spiral grooves. Larvae are white and legless and have a reddish-brown head.

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.

Management Strategies:

  • No chemical control is available.
  • Rotation of alfalfa with non-host crops breaks the cycle and decreases the population of snout beetle significantly as long as neighbouring fields are also rotated out of alfalfa during that time.
  • Non-host crops include corn, small grain cereals and soybeans. In heavily infested fields, it is recommended that the rotation crops be grown for at least 2 years to reduce the insect population dramatically.
  • To minimize the chance of transporting this pest, clean all farm machinery thoroughly before moving it from an infested field.
  • Store first-cut hay from infested fields for at least 2 months before it is shipped. There are restrictions on the transportation of hay, straw, soil and compost material from infested fields.
  • If this pest is suspected, consult the provincial field crop entomologist, field crop IPM specialist or forage specialist.
  • Parasitic nematodes have been found to be effective at collapsing populations in research trials in New York.

Alfalfa Blotch Leafminer (Agromyza frontella)

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.
Life History: In late May, the adult fly emerges from pupa over-wintering on the soil surface. The female adult lays her eggs inside the leaves of new alfalfa plants. The larvae develop inside small tunnels in the leaves. Larvae drop to the ground when mature and pupate. A second generation of adults appear in approximately 1 week (mid-July) and a third generation appears in mid-August.

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.

Plate 91. Alfalfa blotch leafminer larvae feed inside the leaflet, creating tunnels or mines between the top and bottom layers of the leaf.

Plate 91. Alfalfa blotch leafminer larvae feed inside the leaflet, creating tunnels or mines between the top and bottom layers of the leaf.

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.

Management Strategies:

  • A species of parasite successfully controls the alfalfa blotch leafminer in Southern Ontario.
  • Insecticides are harmful to this parasite and, therefore, are not recommended unless leafminer populations are extremely high.
  • For insecticides to be effective, apply them no later than the pinhole stage of feeding.

Alfalfa Weevil (Hypera postica)

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.

Plate 92. Alfalfa weevil adults are about 8 mm long. Note the dark stripe running along the back.

Plate 92. Alfalfa weevil adults are about 8 mm long. Note the dark stripe running along the back.

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).

Plate 93. Alfalfa weevil larvae feed on leaves, causing skeletonization. Note that larvae have black heads.

Plate 93. Alfalfa weevil larvae feed on leaves, causing skeletonization. Note that larvae have black heads.


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.

Action Threshold:

  • Leaf tip damage and weevil counts are used in assessing threshold levels and appropriate action of either cutting or insecticide application. If there is 40% leaf-tip feeding, with 2 or 3 active weevils per stem, and there is more than 7-10 days to preferred harvest date, consider applying an insecticide. ("Leaf-tip feeding" refers to the percent of plant tips showing obvious signs of damage, which is not to be confused with the percent defoliation.)
  • Less than 1 active larva per stem does not require action, but continue to monitor the situation.
  • Two larvae per stem requires action if the alfalfa is less than 40 cm (16 in.) high.
  • If there are more than 3 active larvae, immediate action is required.

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 with insecticide.

Management Strategies:

  • Insecticides are recommended only when cutting is impractical, such as when the alfalfa is in the pre-bud stage. Cutting before the bud stage may result in reduced alfalfa vigour and excessive forage quality for most livestock. It can result in reduced yields due to extensive weevil damage to second cut regrowth.
  • The key to weevil control is proper timing of harvest or insecticide application based on field inspection. When threatening infestations occur, cut fields immediately to eliminate feeding damage. Most of the larvae will be removed from the field, while any remaining larvae usually dry out, starve and are exposed to natural enemies.
  • Occasionally, warm May weather will result in an early hatch of weevil. Feeding damage will show before the bud stage when it would be practical to harvest the alfalfa. In those situations, an insecticide may be warranted.
  • Contact local beekeepers so they have an opportunity to move colonies out of the danger area. It should be noted that spraying insecticides on alfalfa will also kill beneficial insects, the natural enemies of alfalfa pests. This increases the potential for future outbreaks of these pests.
  • Alfalfa weevil is also controlled by a fungal pathogen, Zoophthora phythonomi. Rain and high humidity during periods of peak larval numbers can cause the weevil population to crash quickly. Infected larvae curl around leaves at the tops of plants and then turn brown.

True Armyworm (Pseudaletia unipuncta)

True armyworm is an important pest of forages. For information on this insect.

Potato Leafhopper (Empoasca fabae)

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 are wingless.

Plate 94. Potato leafhoppers are pale green, wedge-shaped, winged insects. Immature nymphs are smaller and wingless.

Plate 94. Potato leafhoppers are pale green, wedge-shaped, winged insects. Immature nymphs are smaller and wingless.

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."

Plate 95. Potato leafhopper injury, "hopper burn," is most severe in new seedlings and in young growth.

Plate 95. Potato leafhopper injury, "hopper burn," is most severe in new seedlings and in young growth.

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, drier-than-normal seasons.

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.

Table 13-5. Action Thresholds For Potato Leafhopper
on Alfalfa and Dry Edible Beans
Action Thresholds
Alfalfa
Dry Edible Beans
Stem Height3 Number of PLHs per Sweep1 Bean Growth Stage PLH Threshold/ Trifoliate2
9 cm (3.5 in.)
0.2 adults
unifoliate
0.25
15 cm (6 in.)
0.5 adults
2nd trifoliate
0.5
25 cm (10 in.)
1.0 adults or nymph
4th trifoliate
1.0
36 cm (14 in.)
2.0 adults or nymph
first bloom
2.0
1 1 sweep = 180° arc.
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:

  • See Table 13-5, Action Thresholds For Potato Leafhopper on Alfalfa and Dry Edible Beans.
  • Resistant varieties are available that use glandular hairs as the resistance factor. These glandular hairs, both on the leaves and stems, act as mechanical barriers to PLH feeding. The glandular hairs are not fully expressed the first year.
  • Treat new seedings of PLH-resistant varieties the same as regular alfalfa.
  • When considering whether to use a PLH-resistant variety, include level of PLH infestation expected in a typical year (higher in Lake Erie counties), cost of scouting, insecticide and spraying, any additional cost of PLH-resistant varieties and other performance traits of the variety, such as yield and disease resistance.
  • Cutting alfalfa early will potentially reduce egg, nymph and adult populations. A naturally occurring fungal pathogen helps reduce the populations of the PLH under cool, moist conditions.
  • Predators and parasites appear to play a minor role in controlling the pest. If populations exceed the action thresholds, an insecticide may be necessary.
  • Spraying insecticides on alfalfa will also kill beneficial insects, the natural enemies of PLH, alfalfa weevil and lygus bug.

Management Strategies for Dry Edible Beans:

  • See Table 13-5, Action Thresholds For Potato Leafhopper on Alfalfa and Dry Edible Beans.
  • Research at the Huron Research Station of the University of Guelph has shown insecticide seed treatment to last at least 4-6 weeks after planting, eliminating the need for at least one foliar insecticide application.
  • Consider using insecticide seed treatment on fields with a history of leafhopper infestations, to reduce the number of foliar applications required.
  • Use foliar insecticides if thresholds have been reached. A naturally occurring fungal pathogen helps reduce the populations of the PLH under cool, moist conditions. Predators and parasites appear to play a minor role in controlling the pest. If spraying during bloom, spray in the evening when bees are less active and contact local beekeepers so they can protect their hives. Rotate insecticide chemical families to reduce the risk of resistance.

European Skipper (Thymelicus lineola)

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:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 30 April 2009
Last Reviewed: 18 August 2009