Managing insect pests in sorghum

Microplitis cocoon attached to a dying helicoverpa caterpillar on a sorghum leaf.

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• pre-planting
• planting to flowering
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'Long term profit will be ahead if growers and advisers plan ahead for insect management in sorghum. Forward planning an Integrated Pest Management (IPM) approach can help minimise use of broad-spectrum insecticides and capture the benefits of natural beneficial insect activity in crop, rather than reacting to problems after they arise,' said Dr Adam Hardy, Senior Entomologist at the Department of Primary Industries and Fisheries (DPI&F) Toowoomba.

Suggestions for managing insects at various stages during the crop include:

Soil insect pests such as wireworms, crickets and earwigs may be in high enough numbers to affect seedling establishment. Dr Hardy said: 'Seed treatments are now commonly available in sorghum and may be the most effective control as well as the least disruptive to beneficial insects.'

Sorghum midge (Stenodiplosis sorghicola) is best managed by planting resistant varieties. Early planted crops with moderate midge ratings (3-5) are unlikely to record economic damage. However any crop likely to flower 2-4 weeks after neighbouring crops may be at greater risk. 'By using a high midge resistant variety later in the season, you´re not only protecting that crop but also subsequent crops by reducing midge build up,' Dr Hardy said.

Aphids and Rutherglen bugs prefer compact or closed panicle types. With the difficulty of achieving spray penetration on a closed-head variety, choosing open-headed type sorghum hybrids is wise.

Few insects commonly cause economic damage during the vegetative growth stages. The corn aphid is the most common, however beneficial insects such as hoverflies, ladybirds, and wasp parasites control them and will move from crop to crop, provided broad-spectrum insecticides are not used. A good rainfall event is usually more effective than any chemical application.

Daily monitoring from head emergence through to early grain fill is needed. At this stage the crop is most at risk of economic damage by two major insect pests - corn earworm (Helicoverpa armigera) and the sorghum midge (Stenodiplosis sorghicola).

Helicoverpa eggs are commonly laid at head emergence. Using the non-disruptive biological virus spray Helicoverpa nucleopolyhedrovirus (NPV) to control helicoverpa helps beneficial insects such as trichogramma wasps and microplitis to survive and to maintain a natural suppression of pest insects. Trichogramma are tiny wasps that can parasitise 60-100% of helicoverpa eggs. Microplitis prefers to parasitise late first and second instar helicoverpa larvae, often achieving parasitism levels of 30-50%. Early application of NPV is often better, as high levels of secondary NPV infection have been recorded.

It is critical that most larvae are 7-13mm or less when spraying NPV so regular monitoring is very important from flowering onwards. New data shows using NPV is harmless to humans and beneficial insects but kills helicoverpa larvae.

How much economic damage does helicoverpa cause? New data shows that one larvae can consume 2.4g of grain in its lifetime if not controlled. One larvae per metre is thus about 24 kg of grain/ha.

Sorghum midge attacks just prior to and during flowering. Hybrids with an 8+ rating do not sustain significant yield loss under midge pressures of 10-20 sorghum midge per day. However these thresholds are reduced by many times in 1-7 rated hybrids. 'By choosing a variety with as high a rating as possible, growers can often avoid the need for highly disruptive synthetic pyrethroids (SPs). While the SPs are relatively cheap to use, they can wind up costing more over time as they kill all the beneficial insects - leading to flares in the helicoverpa and aphid populations,' Dr Hardy said.

Aphid and Rutherglen bug are becoming more common pests during late grain fill, but rarely cause direct yield loss. Aphids can however cause honeydew problems and sticky grain at harvest. Rain or overhead irrigation is the best cure for reducing their sticky exudates. High densities of Rutherglen bug crushed in headers can also cause harvest problems. One well-timed spray on populations that reach 20-30 insects per panicle will ensure that population outbreaks do not occur.

'Overall sorghum crops may be profitably grown in most seasons without the use of insecticides. Under this situation growers may be able to consistently grow sorghum crops that are both good for the environment and beneficial to surrounding crops as beneficial insects move into neighbouring fields,' Dr Hardy said.

Possibly the most important thing to consider this year is the effect the high price of grain may have on calculating economic action thresholds for insect control. At $300 or $400/tonne much lower insect pressures will cause greater dollar losses. Dr Hardy suggests: 'Growers should have an economic threshold calculated in advance that incorporates economic and IPM benefits and then monitor crops very closely, particularly during head emergence and flowering.'

See sorghum insect photo gallery