Insect pest management in sorghum
Major pests of sorghum
Sorghum is susceptible to insect pests from emergence to late grain fill. Early sorghum pests include armyworms and soil insects. These pests are normally present in a grain sorghum crop in low numbers where their damage can be tolerated. However, seasonal conditions can sometimes stimulate the build up of a large population of one or more of these and they can cause significant damage.
Seed dressings to combat soil insects are now commonly available and may be the most effective control as well as the least disruptive to natural enemies.
Pests, such as aphids and rutherglen bugs, can be deterred by choosing open-headed type sorghum hybrids. These insects have a preference for compact or closed panicle types on which they are hard to control due to the difficulty of achieving spray penetration.
It is now possible to adopt an integrated pest management (IPM) strategy to control helicoverpa and midge on sorghum through the use of the Nucleopolyhedrovirus (NPV) that is selective for helicoverpa, and by planting of midge-tolerant hybrids. Such a strategy eliminates any impact on natural enemies that naturally attack both midge and helicoverpa.
Major pests of sorghum
Helicoverpa armigera do little economic damage as foliage feeders. Eggs laid on heads just prior to flowering produce larvae that cause economic damage. Sorghum is most at risk from helicoverpa from head emergence to early grain fill. Larvae of helicoverpa can be confused with sorghum head caterpillar and yellow peach moth.
- Small larvae (less than 10 mm long) feed on the pollen sacs in the flower head where they cause little damage.
- Larger helicoverpa larvae feed on developing seed.
- Each larva destroys about 2.4 gm of grain in its lifetime.
- Monitor weekly from head emergence through to early grain.
- Determine helicoverpa numbers by rotating five head stalks into a bucket.
- Count larvae in the bucket and work out an average per head.
- Determine larval sizes (important for control decisions).
- Repeat sampling at a minimum of six sites throughout the paddock.
- Action levels vary with factors such as grain prices and cost of control. The critical number of insects per head is the number of insects that can eat more dollars worth of grain than the cost of spraying. Control is recommended when insect numbers are at or above the critical threshold level.
- The critical helicoverpa larvae number can be calculated with the following formula:
- C (cost of control) x R (row spacing) divided by V (value of crop in $) x N (no. of heads/m row) x 2.4
As a guide, one or two larvae per head may result in economic damage.
The best product to use for control of helicoverpa is a naturally occurring virus that targets helicoverpa. There are a number of commercially formulated NPV products on the market that are cost-effective for the control of helicoverpa.
- NPV is most effective against small larvae (less than 7 mm). Avoid targeting larvae over 13 mm.
- Crop coverage is critical as this is an ingestion product.
- Spray NPV when about 50% of the sorghum panicles have completed flowering (see label for instructions).
- Spray when larvae are actively feeding between 25oC and 35oC.
- NPV is harmless to wildlife and natural enemies of helicoverpa.
- Use of additives with NPV will improve performance.
There are other chemical means of controlling helicoverpa; however, these chemicals severely disrupt natural enemies. For current chemical control options see Pest Genie or Australian Pesticides and Veterinary Medicines Authority, (APVMA).
The combined action of natural enemies can have a significant impact on potentially damaging helicoverpa populations. It is therefore desirable to conserve as many of the natural enemies as possible. Natural enemies of helicoverpa include predators of eggs, larvae and pupae, parasites of eggs and larvae and caterpillar diseases.
Predatory bugs and beetles that attack helicoverpa eggs and larvae include:
- spined predatory bugs
- glossy shield bugs
- damsel bugs
- bigeyed bugs
- apple dimpling bugs
- assassin bugs
- red and blue beetles
- predatory ladybird beetles
- Trichogramma spp - tiny egg parasite wasps
- Microplitis and Netelia (wasps) - caterpillar parasites
- Species of tachinid flies - caterpillar parasites
With the exception of Trichogramma and Microplitis, most parasites do not kill helicoverpa until they reach the pupal stage. Predatory earwigs and wireworm larvae are significant predators of helicoverpa pupae.
Sorghum midge - Stenodiplosis sorghicola
Sorghum midge is a serious pest of sorghum that requires several repeat insecticide applications during flowering, particularly for late planted crops. Management is now centred on growing midge resistant hybrids that not only simplify management and decreases cost, but provide greater midge control.
- Midge eggs hatch into transparent white larvae that feed on immature seed.
- Feeding creates a depression in the developing seed and prevents seed kernel development.
- High populations of midge can completely destroy the crop.
- The progeny of each egg-laying adult can destroy 1.4 gm of grain.
- Sorghum midge damage is evident by white pupal cases that stick out of the tips of glumes.
The midge lifecycle varies from 2-4 weeks, depending on temperatures. This allows many generations to occur in one season and accounts for the rapid build-up of extremely high midge densities especially where the flowering period of sorghum is extended by successive plantings
Count adult midge on flowering heads at mid morning. Repeat daily.
Thresholds vary with the resistance levels of the hybrids as well as commodity prices and the cost of insecticides. Threshold levels can be calculated using the factor of 1.4 gm of grain destroyed per one egg-laying adult. On susceptible hybrids this level is usually about one adult per head.
The most common means of controlling sorghum midge is through the use of resistant hybrids. Since 1993 all commercial sorghum hybrids have been assigned official midge resistant (MR) ratings from 1-7.
A 7-rated hybrid, when exposed to the same midge density as the susceptible hybrid (rated 1), sustains 7 times less damage. In 2002 the rating system was extended to a new 'open-ended' rating of 8+. Trials have shown that some 8+ hybrids contain levels of resistance that approach 'practical field immunity'. It is worth noting that for 8+ varieties, some are just a little better than 7 while others are 'practically immune'
Today, over 99% of grain sorghum in Australia has some level of midge resistance with most commercial hybrids rating from 4-6. The high level of adoption of MR cultivars and the elimination of low rated MR hybrids means that spraying for midge is now very rare with less than 5% of crops treated, in contrast to the mid 1990s when 30-40% of the crops were sprayed. The use of resistant hybrids also means that natural enemies are conserved.
It is recommended that producers use resistant sorghum hybrids to combat sorghum midge.
Three small black wasp parasitoids play a role in the control of sorghum midge. They are: Eupelmus sp., Tetrastichus sp. and Aprostocetus sp. Their presence in sorghum crops may be recognised by their small round emergence holes in the spikelet.
False wireworms, striate, eastern and southern Pterohelaeus and Gonocephalum spp.
- Larvae feed on decaying vegetable and crop residues in the soil.
- They also feed on newly germinating seed and the growing points of seedlings, which results in patchy stands.
- Damage is most common in early planted crops where crop residue has become scarce.
- During summer, adults may damage young plants, by surface feeding or cutting of the plant at or near soil level.
- Damage by both larvae and adults may necessitate re-planting.
The risk from adults is highest in summer. For larvae the risk is highest for early (September-October) planted crops. Damage may occur if early plant growth is slowed by cool damp weather allowing larvae to remain in the moist root zone. As soil dries they retreat below the root zone.
Monitoring and thresholds
Detection can be difficult - either hand sift 10 soil samples (30 x 30 cm) or place 10 germinating seed baits (GSB) throughout the paddock. One larva per sample warrants control. See monitoring for soil insects.
High mortality of false wireworms can be caused by cool wet weather from autumn to spring. False wireworm beetles are more damaging to seedlings where stubble is buried by cultivation than in crops that are directly drilled into the surface retained stubble. This is because the surface feeding beetles remain feeding on the stubble and not the crop.
- Prepare ground for even and rapid germination.
- Use of press wheels at planting provides some control.
- Clean cultivation during summer dries out topsoil and eliminates weeds that provide food for adults.
- Larvae can be controlled by insecticide applications at planting or insecticide treated seed.
- Control of adults is obtained by baiting with insecticide treated cracked grain broadcast evenly over the surface at or immediately after planting.
- Where broadcasting is not possible, the bait may be laid in trials spaced no more than 2 metres apart. For current chemical control options see Pest Genie or APVMA.
Natural enemies provide little control.
Cutworms - Agrostis spp.
- Brown cutworm - Agrotis munda, Bogong moth - Agrotis infusa, Black cutworm - Agrotis ipsilon and Variable cutworm - Agrotis prophyricollis
The common name of cutworm is derived from the larval habit of severing the stems of young seedlings at or near ground level, causing the collapse of the plant. Cutworm larvae typically shelter in the soil during the day. They curl into a 'C' shape when disturbed.
- Cutworm larvae can sever stems of young seedlings at or near ground level.
- Sometimes the young plant is partially dragged into the soil where the larvae feed on it.
- Larvae may also climb plants and browse on or cut off leaves.
- Crop areas attacked by cutworms tend to be patchy and the destruction of seedlings in one area may cause cutworms to migrate to adjacent fields.
- Risk period is spring and summer - one generation per crop.
- Weedy fallow prior to sowing can lead to cutworm infestation.
Monitoring and thresholds
- Inspect emerging seedlings twice per week.
- Treat seedlings when there is a rapidly increasing area or proportion of crop damage (greater than 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.
- Cutworms are attacked by a range of natural enemies such as parasitoids, predators and diseases.
- Controlling weeds prior to planting will reduce cutworm infestations.
- Insecticides are used when damage warrants their use. For current chemical control options see Pest Genie or APVMA.
- Spot spraying of identified patches may suffice. For best results, spray late in the afternoon, close to feeding time.
Black field earwig - Nala lividipes
Black field earwigs are a sporadic pest of sorghum, usually in areas with heavy, black soils.
- Eat newly sown and germinating seed and the roots of crops below ground, resulting in poor establishment.
- Chew the stems of newly emerged seedlings above ground.
Monitoring and control
- Monitor crops after planting until establishment.
- Use germinating seed baits or digging and sieving to detect adults and nymphs prior to planting.
- 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.
- Earwigs prefer cultivated soils rather than undisturbed soil (zero til).
- Use press wheels at sowing.
- For current chemical control options see Pest Genie or APVMA.
Armyworm is the caterpillar stage of certain moths and can occur in large numbers especially after good rain following a dry period. During the day armyworms shelter in the throats of plants or in the soil and emerge after sunset to feed. The adults of the common and northern armyworms may be confused. Genitalia dissection by a specialist is required to separate the species. The larval stages likely to be encountered in cereals are all similar in appearance.
- Young plants may be defoliated or killed by larval feeding.
- 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.
- The northern and common armyworm feed at night and hide in vegetation during the day.
- Dayfeeding armyworms feed during the day.
- Visually monitor during the early growth stage and again during head emergence and flowering.
- Egg lays are often associated with heavy rainfall so check for larvae several weeks after rainfall events.
- Since armyworms (except dayfeeding armyworm) hide during the day, look under clods of soil, under vegetation and at the base of plants. Also look for dark green-brown faecal pellets.
Treatment decisions should be made on whether the crop can compensate for damage, the prevalence of natural enemies, the value of the crop and the cost of treatment. As a guide, one-two caterpillars/m2 may warrant control.
- Many chemicals will control armyworms. Their effectiveness is often dependent on good penetration into the crop to get contact with the caterpillars.
- 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. For current chemical control options see APVMA
Armyworm larvae are attacked by a number of parasitoids that may be important in reducing the intensity of outbreaks. When armyworms are in numbers likely to cause damage, parasitoids are unlikely to give timely control.
Predators include green scarab beetles, predatory shield bugs and perhaps common brown earwigs. Viral and fungal diseases are recorded as causing mortality of armyworm.
Corn aphid - Rhophalosiphum maidis
Although other species of aphids may be present on sorghum, the corn aphid is the most common and numerous.
- Adults and nymphs suck sap and produce honeydew.
- High numbers can cause plants to turn yellow and appear unthrifty.
- High populations on heads produce sticky grain and clog harvesters.
- Water stressed dry-land crops loose yield.
- Estimate the percentage of plants infested and the percentage of leaf area covered by aphids.
- The action level in the vegetative stage is 100% of plants with 80% of the leaf area covered by aphids. On the heads it is 75% of heads with 50% of the head covered by aphids.
- Choosing hybrids with open heads can reduce aphid numbers as these are generally less infested than tight-headed hybrids.
- Chemical control options are cost effective but all insecticides that control aphids impact on natural enemies. For current chemical control options see Pest Genie or APVMA.
A range of parasitoids and predators will help reduce aphid populations. Predators of aphids include:
- ladybird larvae
- damsel bugs
- bigeyed bugs
- larvae of green lacewings
- larvae of hoverflies
- Wasp parasitoids such as Lysiphlebus testaceipes mummify and kill aphids.
- Seed heads may be damaged by feeding, resulting in reduced yield and quality.
- Affected seed is red and spotty and hollowed out internally.
- Grain will be small and shrivelled and does not continue to fill beyond the point it was damaged.
- Damaged seed is subject to fungal and bacterial attack.
- Monitoring for RGB can be done at the same time as for helicoverpa.
- Monitoring methods consist of beating sorghum heads into a bucket and counting the insect pests.
- Because RGB can start to build up at any stage of crop maturity and impact on seed set during flowering and early seed development, monitoring should start at early flowering.
- Distribution is typically patchy across the field which means that the more samples taken, the greater the level of confidence in the overall infestation.
- Preliminary research on RGB has shown that adult RGB will reduce seed set by around 20% at densities of 50-100 bugs/head.
- Research has indicated that there is no impact on yield or quality once the crop is in the 'hard dough' stage.
- New thresholds for earlier stages are proposed with a threshold during flowering and milky dough stage of greater than 30 bugs/head and a threshold of greater than 80 bugs/head during the soft dough stage.
- More research into the density-damage relationship is required to come up with robust thresholds for RGB in sorghum.
- Control of RGB should occur in the early seed development stages.
- There are no soft chemical options for the control of RGB. Repeated influxes of migrating adults can make repeat applications necessary.
- For current chemical control options see Pest Genie or APVMA.
Egg parasitoids are the most commonly recorded natural enemy of RGB. Their potential contribution to population control will be limited in seasons when there are large influxes of adults. Predation has rarely been recorded, but spiders may play a role.
This pest in more prevalent in tropical and sub-coastal areas of north-eastern Australia. It may be confused with helicoverpa and yellow peach moth.
- Larvae feed on developing seed, each larva destroys anout 0.5 gm of grain.
- They web clusters of seed together.
- Presence of caterpillars is indicated by webbing of seed clusters, webbing of whole heads and presence of small white/pink excreta.
- Count larvae on heads by dislodging them from sorghum heads into a bucket.
- Action levels vary with commodity prices and the cost of insecticides.
- The threshold level can be calculated using the factor of 0.5 g of grain destroyed by the larva.
- The threshold is usually in the range of 5-10 larvae per head.
- Open-headed sorghum varieties are generally less infested that tight-headed varieties and also allow better penetration of insecticides.
- There are currently no registered products against sorghum head caterpillar but chemicals targeting helicoverpa are likely to be effective.
- For chemical control options consult Pest Genie or APVMA.
- Parasitic wasps provide some biological control, but are unlikely to control populations above 10 per square meter.
- The avoidance of broad-spectrum pesticides prior to flowering may help conserve natural enemies.
Yellow peach moth - Conogethes punctiferalis
This pest may be confused with helicoverpa and sorghum head caterpillar.
- Larvae feed on developing seed. Each larva destroys about 1 gm of grain.
- Count larvae during milky dough stage and by dislodging them from heads into a bucket.
- The level varies with factors such as commodity price and cost of insecticide. The level can be calculated using the factor of 1 gm of grain destroyed per larva. The threshold usually works out to be 2-5 larvae per head.
- Open-headed sorghum varieties are generally less infested than tight-headed varieties as they allow better penetration of insecticides.
- Chemical control is cost effective. For current control options see Pest Genie or APVMA.
The avoidance of broad-spectrum pesticides prior to flowering may help conserve natural enemies. Parasitic wasps provide some biological control.
- Pests of Field Crops and Pastures: Identification and Control. Editor: P.T. Bailey