Insect pest management in maize
Maize can be attacked by a wide range of insects. The main pests of maize are helicoverpa and a number of soil insects. Minor pests occur irregularly and will not be a problem every season.
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Pests can occur at one or more growth stages (Insects that can potentially be a major pest are indicated in bold):
| black field earwig|
true wireworms maize leafhopper
swarming leaf beetles
dayfeeding armyworm corn aphid
green vegetable bug
redshouldered leaf beetle
|corn earworm|| twospotted mite
green vegetable bug|
redbanded shield bug
yellow peach moth
Helicoverpa armigera is major, widespread, regular pest. Female moths lay eggs on the stem, leaves (both sides) tassels, silks and husks on the upper two-thirds of plants. Caterpillars hatching prior to silking cause little damage to tassels but may cause damage when migrating to cobs. Larvae from eggs laid on silks or husks may cause significant damage. Silk damage reduces pollination and grain-set. Feeding damage also occurs on the top 1-3 cm of the cob, and may result in the presence of mycotoxins.
Leaf damage can indicate pest presence. Parallel rows of holes are signs of feeding on unopened leaves. Helicoverpa are usually not considered economical to control, except in high value seed maize. Note that helicoverpa larvae may be confused with armyworms or cutworms.
Maize varieties with husks extending 50-80 mm beyond the top of the cob and closing tightly around the silks restrict the entry of larvae into the cob. Watering during dry weather prevents the husks from loosening.
Maize crops often have high levels of beneficial insects (predators and parasitoids) that may be harmed by insecticide applications. The combined action of natural enemies (including predators of eggs, larvae and pupae, parasites of eggs and larvae, and caterpillar diseases) can have a significant impact. Cultivation to a dept of 100 mm destroys overwintering pupae.
Chemical control should target small caterpillars (up to 5 mm) and be directed at tassels and emerging silks. The best product to use in an integrated pest management system is a naturally occurring nucleopolyhedovirus (NPV). There are a number of commercially formulated NPV products on the market for the control of helicoverpa. For more information, visit the NPV page .
Soil-dwelling insect pests can seriously reduce plant establishment, plant populations, plant growth, and subsequent yield potential, and should be monitored for prior to plating. Seed treatments help deter feeding. In-furrow spraying helps protect young roots and shoots. Press wheels can reduce damage from false wireworm larvae and earwigs by encouraging plant emergence and firming the soil to reduce insects´ ability to move through it. Soil baiting may reduce damage by black field earwigs and crickets that attack the tips of developing prop-roots (secondary roots). Shallow planting in warm moist soil will encourage rapid crop emergence and growth, thus reducing the impact of insects. For general information about soil insects and germinating seed baits (GSB) see How to monitor for and recognise soil insects . Black field earwig is a sporadic and potentially major pest of maize, black earwigs eat newly sown and germinating seed and the roots of crops resulting in poor establishment. Feeding on secondary roots may cause the plants to fall over as they get larger. Serious damage is usually confined to soils that retain moisture well, and earwigs prefer cultivated soils to undisturbed soil (zero till). Note: predatory earwigs are usually larger and light brown in colour.
Monitor crops after planting until establishment. Dig and sieve soil to detect adults and nymphs prior to planting. Use germinating seed baits and control if more than 50 earwigs in 20 germinating seed baits. Grain baits containing insecticide applied at sowing offer best protection. Insecticide seed dressings provide some protection. In-furrow sprays are not effective in protecting against dense populations. Use press wheels at sowing.
True wireworm larvae bore into germinating seed and chew on seedling roots and shoots resulting in reduced vigour, wilting or seedling death. Damage is worse when crop growth is retarded by dry, wet or cool conditions. Wireworms generally favour moist areas. True wireworm larvae may also feed on helicoverpa pupae.
Use germinating seed baits or soil sampling to detect larvae prior to sowing. Monitor crops after sowing until establishment. One larvae/germinating seed bait warrants control. Seed dressings, in-furrow sprays and granular insecticides offer some control.
False wireworm larvae attack germinating seeds and seedling roots and shoots in spring, resulting in patchy stands. Damage is most common in early planted crops with low crop residue (e.g. cultivated paddocks). Adults may damage summer seedlings by chewing at or above ground level and replanting may be required.
To detect, either hand sift 10 soil samples (30 x 30 cm) or place 10 germinating seed baits throughout the paddock. One larva per sample warrants control. Prepare ground for even and rapid germination. Use of press wheels at planting provides some control. For larvae, use seed treatments or in-furrow sprays. For adults, use cracked grain baits. Natural enemies provide little control. Infestationsdtected after crop emergence cannot be controlled. Cutworm larvae feed on leaves and stems of young plants, and ´cut´ down plants to eat the leaves. Partial damage to stems may cause the plant to wilt. Larvae typically shelter in the soil during the day and curl into a ´C´ shape when disturbed. Cutworms may be found in any soil type and often move into the crop from adjoining fence lines, pastures or weedy fallows. Crop areas attacked by cutworms tend to be patchy and the highest risk period is during summer and spring.
Inspect emerging seedlings twice per week. Treat seedlings when there is a rapidly increasing area of infestation or proportion of crop damage (>10% seedling loss). Treat older plants if more than 90% of plants are infested or more than 50% of plants have 75% or more leaf tissue loss. Spot treatments (e.g. along field edges) may be successful. Spray when caterpillars are feeding (dusk-night). Keep fallows clean and eliminate weeds from paddock perimeters for at least one month before planting. Severe damage to emerging crop can occur when large larvae are forced to move from weed hosts into the crop following spraying of the weeds. Cutworms are attacked by a range of natural enemies such as parasitoids, predators and diseases. Whitegrub larvae (
spp., andPseudoheteronyx spp) feed on roots causing loss of vigour and lodging. Adults may feed on leaves. Damage is often patchy, and there are no effective controls, although a
Metarhiziumfungus and entomopathogenic nematodes have been reported to occasionally cause high larval mortality. Avoid sowing new ground with maize after pasture in areas that have a known history of white grubs.
Maize leafhoppers and maize thrips are widespread but irregular in Queensland, and can rapidly re-infest crops after spraying meaning more than one spray may be required.
- Maize leafhoppers are a pest of germinating maize. Most common during late summer, they suck sap and high populations (>15/plant) can transmit wallaby ear mycoplasma. Mycoplasma-infected plants are dark green colour with thickened veins on the underside of leaves. Hybrid varieties offer some resistance to wallaby ear. Inspect crops weekly during the vegetative stage, and control if more than 10 leafhoppers/plant and wallaby ear symptoms are present.
- Maize thrips attack seedling and vegetative growth stages. Thrips in the whorl can stop the growth of small plants. Infected plants may have a yellow or silvery patches on the leaves and a desiccated or wilted appearance. Damage is more severe if plants are stressed by drought or water logging. Inspect weekly during seedling and vegetative stages, and control if thrips are found in the throat combined with yellowing of the throat or necrotic stripes on young leaves.
Corn aphid is the most common aphid species on maize and can affect any crop stage. Adults and nymphs suck sap and produce honeydew. High numbers can cause plants to turn yellow and appear unthrifty. Yield loss may occur on water stressed plants. The incidence of damage is generally too low to warrant control. Chemical control options are generally not cost effective and the insecticides that control aphids may negatively impact natural enemies. Inspect at weekly intervals. Predators of aphids include ladybird larvae, damsel bugs, bigeyed bugs, larvae of green lacewings and larvae of hoverflies. Wasp parasitoids mummify and kill aphids.
Green vegetable bugs (GVB) are widespread but irregular pests of maize during summer. Adults and nymphs feed by piercing and sucking on developing cobs, and may severely deform cobs. Although chemical control may be cost effective, there are currently no threshold levels for GVB in maize.
Two-spotted mite is a widespread but irregular pest of maize during seed fill to maturity. Mites are usually present towards the end of the crop cycle during late summer/autumn and are favoured by hot, dry weather. Adults and nymphs pierce and suck on lower leaf surfaces, causing yellowing on the upper leaf surfaces. Fine webbing on the lower leaf surface indicates their presence, and heavy infestations will result in leaf desiccation, leaf drop and yield loss. Initial infestations can be patchy. No thresholds are available for mites in maize and control is not cost effective. Conserve natural enemies through reduction of broad-spectrum insecticides against other pests as the disruption of natural enemies can flare mite outbreaks.
Redbanded shield bug is a widespread but minor summer/autumn pest that can be confused with green vegetable bug. Adults and nymphs feed by piercing and sucking on developing cobs, causing deformities, similar to green vegetable bug damage.
Locusts are sporadic and potentially major pests of maize. Adults and hoppers chew irregular pieces from leaves and stems and can cause complete defoliation overnight if populations are high enough. Species found in Queensland include: Australian plague locusts, Migratory locusts, and Spur-throated locusts. The Australian Plague Locust Commission provides details of hopper migrations.
Whitefringed weevil larvae chew into lateral roots causing death and reduced vigour. Infestations are usually patchy. Mass emergence of adults occurs after rain in November-January, and damage is often worse when two host crops (e.g. maize, peanuts, chickpea, lucerne) are grown in sequence.
Leaf beetles are an irregular pest of maize:
- The redshouldered leaf beetle is restricted to coastal areas and can infest at any stage of crop growth. Swarms of adult beetles move into a crop and feed on foliage, tassels, silks and the husk at the top of the cob. Injury to silks may reduce seed set. Feeding exposes the cobs to other insects and diseases. Infestations tend to be patchy so thorough weekly checking is required. Control is warranted if 95% of plants in an area are infested and 70% of flag leaves are eaten. Chemical control is cost effective at high infestations.
- Swarming leaf beetles are a minor, irregular pest in Queensland. The black leaf beetle can be a major pest in northern areas. Leaf beetles are likely to be present during the seedling stage. Larvae feed on roots causing up to 40% seedling death. There are currently no recommended methods of monitoring and economic injury levels have not yet been established. Avoid planting maize immediately after grass pasture.
Armyworms can occur in large numbers especially when good rain follows a dry period. During the day the common armyworm shelters in the throats of plants or in the soil and emerge after sunset to feed. Dayfeeding armyworm are active during the day. Young plants may be defoliated or killed. Older plants can outgrow damage but seed yield may be reduced. Signs of damage include chewed leaf margins and faecal pellets at the base of young plants or in the throats of older plants. Monitor during seedling and vegetative stages. Egg lays are often associated with heavy rainfall so check for larvae several weeks after rainfall events. Look for larvae under clods of soil, under vegetation and at the base of plants. As a guide, control is warranted if out of a count of 30 plants, 27 are infested, and more than 21 have at least 75% flag leaf loss.
Many chemicals will control armyworms but their effectiveness is often dependent on good penetration and control may be more difficult in high-yielding thick canopy crops, particularly when larvae are resting under leaf litter at the base of plants. As larvae are most active at night, spraying in the afternoon or evening may produce the best results. Armyworm larvae are attacked by a number of parasitoids that may assist in reducing the intensity of outbreaks, although are unlikely to give timely control if armyworm numbers are high. Predators include green carab beetles, predatory shield bugs and perhaps common brown earwigs. Viral and fungal diseases are recorded as causing mortality of armyworms.
Yellow peach moth is a minor and irregular pest of maize in Queensland. Larvae tunnel into stems or cobs producing masses of webbing and excreta at the tunnel entrance. There are no chemical controls available. The habit of mining into stems and cobs makes spray application ineffective as larvae are not exposed to insecticide. They are similar in appearance to pink-spotted bollworm larvae.
More detail on these pests can be found in Pests of field crops and pastures: identification and control edited by P.T. Bailey, CSIRO Publishing 2007.