Insects in field crops - Aphids, armyworms, cutworms, mites
On this page:
Aphids
Armyworms
Cutworms
Mites
Checking/scouting methods
Which insects to check for
Beneficial insects and what they eat
Aphids in field crops
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Cotton aphid (Aphis gossypii) |
A wide range of aphid species can affect field crops. Most are small (only a couple of millimeters long) with oval-shaped greenish, brownish or blackish bodies. Often occuring in colonies, aphids suck on sap, causing loss of vigour, and in some cases yellowing, stunting or distortion of plant parts. Honeydew (unused sap) secreted by the insects can cause sooty mould to develop on leaves. In crops such as cotton, the honeydew affects quality. Aphids can also be vectors (carriers) for viruses.
Their feeding rarely causes major damage in most crops, and control measures are usually not warranted, as a range of parasites and predators keep population numbers down. Exceptions may be where the crop is under moisture stress, or where very heavy populations are observed (high populations aggravate the effect of moisture stress). For example, between 30 to 50 aphids per plant could be used as a threshold level for wheat.
An excellent reference with colour photographs for ease of identification is the TOPCROP publication Crop Insects: The ute guide (northern grain belt edition).
Common armyworm in winter cereals
The common armyworm (Leucania convecta ) is important in southern Queensland where it attackes winter cereals, particularly barley and oats, in September and October. Larvae appear in plague proportions in some years, and are patchy in others. Conditions leading to outbreaks are not yet fully understood. Leafy cereal plants can tolerate considerable feeding from moderate numbers of armyworms. Control in the vegetative stage is seldom warranted unless large numbers of armyworms are well distributed throughout the crop and are destroying young seedlings or the younger two or three leaves from older plants.
Infestations within padocks are often patchy and more luxuriant stands often have the highest pest pressure - up to 60 or more armyworms per square metre. Larvae hide in leaf debris or in cracks in the soil and emerge at night to feed. Infestations are indicated by the eaten-out margins of leaves and also by the faecal pellets around the plant base. Older larvae can feed on the green stem just below maturing heads, severing them.
The most serious armyworm damage in cereal crops occurs when it starts feeding on the upper flag leaf and stem node as the crop matures. Head cutting begins at this time. Control is warranted if the armyworm population distributed throughout the crop has potential to cause the loss of 15 heads per square metre. Populations in excess of 1 larva per square metre will usually warrant spraying. For insecticide application to be economic, check or scout the crop and assess the problem before head cutting starts. Check for larvae on the plant and in the soil litter under the plant. Only large larvae can chew through the stem.
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Common armyworm larvae |
Head-cutting damage to barley |
Armyworm can be parasitised by a small wasp (Apanteles spp) - wasp larvae can be seen developing within the caterpillar if it is broken open, or pupating wasps are visible as cocoons attached to the caterpillar's body.
Cutworms (Agrostis spp)
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Cutworm larvae and damage to sunflower seedlings. |
Several species of cutworms attack a wide range of vegetable, field and cereal crops in Queensland. Crops should be monitored from emergence through to establishment. "Cutworm" is derived from the larval habit of severing the stems of young seedlings at or near ground level, causing collapse of the plant. Sometimes, the young plant is partially dragged into the soil, where it is fed upon by the larvae. Crop areas attacked by cutworm tend to be patchy, and cutworms may migrate to adjacent areas if food becomes scarce.
Adult cutworms are dull stout-bodied moths with a wingspan of about 4 cm. Small pearly-white eggs are generally laid on soil under low growing weeds, but may sometimes be deposited amongst organic material on the soil surface. One cutworm species prefers moist soil for egg-laying. The larvae, which emerge after a few days, feed at night and rest just beneath the surface during the day in the soil. There are five larval stages, lasting 4-6 weeks, and full grown larvae measure up to 5 cm long. The larvae are hairless with dark heads and usually darkish grey or brown coloured bodies, often with longitudinal lines and/or dark spots, and the skin often has a rough, granular texture. The larvae curl up and remain still if picked up.
Mature larvae stop feeding and pupate in an earthern cell in the top 5 cm of soil, often under an object (such as a piece of wood). Female moths are highly prolific, and can lay up to 2400 eggs. The lifecycle from egg to adult takes about 2 months.
Cereal crops adjacent to permanent vegetation (or fields previously sown to grassy sod or legumes) are prone to cutworm attack, as are crops which possess high weed populations. Consequently, farm practices often have a considerable bearing on cutworm incidence. Good weed management in the weeks before planting is essential.
The degree of control by insecticides depends on a number of factors; including larval size, soil penetrability to sprays, rainfall and the residual life of the chemical used; but usually only one application is needed.
Cutworms are attacked by a wide range of parasites, predators and diseases.
Mites in winter cereal crops
Blue oat mites (Penthaleus major) are an important pest of seedling winter cereals. Adults move very quickly when disturbed and are about 2 mm long and are readily recognised by their blue, oval-shaped bodies and distinctive bright red legs. If pale green or greyish irregular patches appear in the crop, check for the presence of blue oat mite at the leaf base. The mites feed on the leaves during late afternoon and through the night, resulting in a silvering of the leaf tips. In severe infestations early in the season, leaf tips wither and the seedlings eventually die.
Damage is most likely during dry seasons where the mite is in large numbers accentuating the effect of moisture stress. Control may be warranted in this situation. Since eggs laid in the soil hibernate throughout the winter, populations of the mite can build up over a number of years and cause severe damage if crops are not rotated.
Brown wheat mites (Petrobia latens) are sporadic pests of winter cereals. Adults are smaller than half a pinhead in size, brown and globe-shaped. Damage is severe only under very dry conditions. Feeding causes a fine mottling of the leaves, making the plants appear drought-affected. Heavily infested crops viewed from a distance appear bronzed or yellowish. Control may be considered if large numbers are concentrated on the flag leaf in times of moisture stress.
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Blue oat mite damage to oats. |
Wheat leaves damaged by blue oat mite (left) and brown wheat mite (right). |
Insect checking methods
When to check
All crops should be scouted for insects at regular intervals, usually once/week prior to flowering and 2-3 times/week post flowering. Check crops regularly and look for eggs, small larvae and nymphs. Look for adults as the crop is disturbed, as they are capable of flying away (refer to the table on scouting winter field crops below for specific crop and insect details).
How to check
Check at least five metres of row at six widely spaced sites in the field each time you inspect.
- Direct observation - look on top of leaves, stems and reproductive structures (buds) for small white-cream or brown coloured eggs, small larvae and nymphs.
- Plant shaking to dislodge insect larvae, nymphs and adults. For pulse crops, shake plants onto a beat sheet placed between the rows to make it easier to find the larvae and nymphs. A standard beat sheet is made from plastic or fertiliser bag material. The dimensions are 1.3m wide by 1.5m deep. Only plants along the central metre of row are sampled (shaken) with a 1 metre long stick. The extra 0.15m on each side catches insects sprayed out sideways when sampling and the sheet's depth allows it to be draped over the adjacent row. This prevents insects escaping through this row.
- Pulling flowers apart to check for small helicoverpa larvae and thrips within the flowers.
Take note of crop development stage, all insects present and stage of growth (i.e. eggs, larvae, nymphs or adults). Also record the number of beneficial insects and parasitism levels present.
Use the "Crop Insects: The Ute Guide, Northern Grain Belt ", to identify insect pests and predators. It is available wherever DPI&F books are sold.
Crop inspections per week
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Crop growth stage | |||
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Crop |
Seedling |
Vegetative |
Flowering |
Pod-grain fill |
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Chickpea |
1 |
1 |
2-3 |
2-3 |
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Faba beans |
1 |
1 |
2-3 |
2-3 |
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Linseed |
1 |
1 |
2-3 |
2-3 |
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Canola |
1 |
1 |
2-3 |
2-3 |
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Wheat/Barley |
1 |
1 |
1-2 |
1 |
Record keeping
Keeping records should be a routine part of insect checking. A record of crop inspections will remind you whether pest numbers are increasing or decreasing and help to decide whether a spray is necessary.
Insect scouting in winter field crops
What insects to check for in which crop
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Insect |
Crop | ||||
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Chickpea |
Wheat/Barley/Canary |
Linseed |
Canola |
Faba beans | |
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Soil Insects | |||||
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Cutworm |
R | R | R | R | R |
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Armyworm |
R | ||||
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False Wireworms |
R | R | R | R | |
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Black Field Earwigs |
|||||
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White Fringed Weevil |
R | ||||
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Preflowering stage | |||||
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Helicoverpa (heliothis) |
R | R | S | S | |
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Jassids |
R | ||||
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Thrips |
S | ||||
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Aphids |
R | R | R | ||
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Blue Oat Mite |
R | ||||
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Brown Oat Mite |
R | ||||
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Loopers |
S | ||||
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Budding, flowering, pod set and pod fill | |||||
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Helicoverpa |
R | R | R | R | R |
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Mirids |
|||||
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Green Vegetable Bug |
R | ||||
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Brown Bean Bug |
R | ||||
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Bean Podborer |
|||||
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Rutherglen Bug |
R | ||||
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Grey cluster Bug |
R | ||||
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Jassids |
|||||
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Thrips |
|||||
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Aphids |
|||||
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Whiteflies |
|||||
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Loopers |
|||||
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Locusts |
S | S | S | S | S |
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Diamond Back Moth |
R | ||||
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Armyworm |
R | ||||
S = sporadic, R = regular
Insects without R or S are not usually a problem in winter field crops.
Beneficial insects for managing pests
Which pests do beneficial insects attack?
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Parasites |
Pests Attacked |
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Ichneumonidae wasps (including orange caterpillar parasite) |
Larva of noctuids* (including helicoverpa, armyworms, cutworms and loopers) |
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Microplitis wasps |
Helicoverpa larvae |
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Tachinid flies |
Larva of noctuids |
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Trichogramma wasps |
Eggs of noctuids |
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Trichopoda flies |
Green Vegetable Bug and green stink bug (adults and 5th instar nymphs) |
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Predatory Insects | |
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Ants |
Eggs and larvae of lepidoptera#, bug nymphs |
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Apple dimpling bug |
Lepidoptera eggs, very small larvae and mites |
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Assassin bug |
Range of insects including helicoverpa and mirids |
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Big eyed bug |
Soft bodied insects, lepidoptera eggs and mites |
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Brown smudge bug |
Lepidoptera eggs, aphids, jassids and mites |
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Common brown earwig |
Larvae and pupae of lepidoptera |
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Damsel bug |
Lepidoptera eggs and larvae, mites and mirid eggs |
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Glossy shield bug |
Larvae of lepidoptera |
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Hoverflies |
Aphids |
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Lacewings |
Aphids, lepidoptera eggs and small larvae |
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Ladybeetles |
Aphids, mites, lepidoptera eggs and small larvae |
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Pirate bugs |
Thrips and lepidoptera eggs |
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Predatory shield bug |
Lepidoptera larvae |
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Red and blue beetle |
Lepidoptera eggs and small helicoverpa larvae |
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Spiders |
Generalist predator, including moths, larvae, eggs, bug nymphs and other spiders |
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Thrips |
Mites |
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Pathogens | |
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Bacteria eg Bacillus thuringiensis |
Lepidoptera |
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Fungal diseases (including Beavueria, Nomuraea rileyi and Metarhizium) |
Helicoverpa, loopers, cluster caterpillars, armyworms and podsucking bugs. |
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NPV (Nuclear Polyhedrosis Viruses) |
Helicoverpa, loopers and cluster caterpillars |
* Noctuids include heliothis, armyworm and loopers.
# Lepidoptera include all moths and butterflies.
