Grey false wireworm and its management in seedling canola

AG1102
Hemantha Rohitha, Horsham and Garry McDonald, Rutherglen

Grey false wireworm (Isopteron punctatissimus) is a beetle (Family: Tenebrionidae) native to Australia. Its spread is reported in New South Wales, Queensland, Victoria and South Australia. It has become an important soil dwelling pest of canola particularly on fine textured soils eg. cracking clay.

Description

Figure 1. Grey false wireworm larva (10 mm)

The grey false wireworm larva grows to about 10 mm long and 1.5 mm wide with a robust black-brown exo-skeleton (Figure 1). It has a characteristic pair of black, up-turned spines on the last segment. Its powerful mouthparts chew and ingest plant debris and plant roots. When the soil is disturbed, larvae remain motionless for a few seconds and then move rapidly out of the disturbed area. In grey or dark soils their slightly reflective bodies may aid detection.

The pupa is 8 mm long and 3 mm wide (Figure 2). It is pure white and has no pupal cocoon. Two white coloured spines on the last abdominal segment are helpful in distinguishing grey false wireworm pupa from pupae of carabid (predator) beetles that occur in soil around the same time. The pupae turn brown when they are about to moult into adults.

Newly emerged adults are light brown in colour and they soon change into dark chocolate brown within a day. Adult grey false wireworm are 8 mm long and 2 mm wide (Figure 3). Compared to the larva, the adult is slow moving.

Damage

Grey false wireworm larvae do not cause damage to cereal or pulse crops, but they have a particular affinity for canola. As canola seeds germinate, the larvae feed on the hypocotyl and the root system of the canola seedlings. With their strong mouthparts, larvae ringbark or sever the stems and roots of the emerging or newly established seedlings. In heavy infestations, major establishment failures in canola can occur. The damage appears as large bare patches in the paddock 3-4 weeks after sowing (Figure 4). Damage to seedling roots early in the season can also cause forked or damaged root systems in mature canola plants, which interferes with the water absorption and plant anchorage. Bare patches will require re-seeding, depending on the size of the area affected. Bare patches not only cause yield losses but they also provide havens for troublesome weeds.

Life cycle

Figure 4. False wireworm damage to canola (notice patchiness in establishment)

Grey false wireworm has a one-year life cycle. Adult females lay eggs in late summer and early autumn. In Victoria, eggs hatch around February-March, and the emerged larvae immediately start feeding on the decaying organic matter in the top 10 mm of soil, depending on the soil moisture status. In dry periods, the larvae survive by moving down the soil profile. In March the larvae are about 3 mm in length. At the sowing time in May, grey false wireworm larvae have usually grown to 5-8 mm in length and are able to cause damage to the emerging canola seedlings.

The larvae are fully grown by early September and pupation takes place in mid September. Pupae occur close to the soil surface and are highly vulnerable to mechanical damage at this time. Pupae do not feed and they move by wiggling through soil if disturbed. Adult beetles emerge in October and November and find shelter under debris and in cracks in soil during the day.

Natural enemies

Figure 5: Geoscaptus laevissimus (up to 25 mm) (both adults and larvae are predators of grey false wireworm)

Carabid beetle adults (eg. Figure 5) and larvae (Figure 6) are important predators of both grey false wireworm larvae and adults. Predation by carabid beetles and larvae helps to reduce the grey false wireworm population so reducing the risk to the emerging canola crop. There are five different carabid beetle species present in Victorian soils. Carabid beetles can be distinguished from the grey false wireworm adults by their large bulging eyes situated on each side of the head. They are shiny dark black in colour but some can be metallic green. The larvae have well-developed legs with prominent mandibles. A pair of processes (urogomphi) can be seen on the last segment of the larva.

Integrated management of grey false wireworm

Figure 6. Carabid larva (up to 25 mm)

The recommended action threshold for grey false wireworm is 50 insects/m2 (speculative estimate). A good understanding about the biology is important for successfully managing grey false wireworm. It is essential to estimate grey false wireworm numbers prior to making decisions on insecticide application, though the processes involved are somewhat laborious. More than one strategy (see below) may be needed to protect the canola seedlings from grey false wireworm damage. Some of these control methods may need to be planned in the previous season, well before sowing canola.

Often natural enemies, other mortality factors and cultural methods may bring grey false wireworm numbers below economic threshold levels. However, if the population density exceeds the threshold, an insecticide treatment may be necessary before planting or severe seedling failure and uneven seedling distribution may result. Since the infestations tend to re-occur in the same region and paddocks, a determined approach over several years is needed to minimise the adult population from areas of infestation.

Detection

Detecting larval population

Estimating numbers of grey false wireworm is an essential decision-making tool for assessing potential larval damage to canola seedlings. High larval populations, if not controlled, can result in major losses of canola seedlings. Once the damage becomes obvious it is too late to treat the crop.

Baiting can be used to detect grey false wireworm larvae in soil. Oat or canola grain (200-300 g) pre-soaked for 24 hours, buried in 5 to 10 random sites in shallow holes (50 mm deep) and covered with 10 mm of soil can be a useful detection technique prior to sowing. After seven days, the soil around the baits should be removed and carefully sorted to check for grey false wireworm larvae. The baiting method will provide evidence of larval presence but does not give an estimate of the larval density.

Carefully count the larvae in the top 20 mm of soil in a 30 cm x 30 cm quadrat (approx 0.1 square metre). Sample 5 to 10 places at random in the paddock. This method will provide an estimate of the larval population density in the paddock. The success of this technique depends on the presence of adequate soil moisture in soil at the time of sampling. In dry soil conditions grey false wireworm larvae move down the soil profile giving an underestimate of the actual larval numbers.

Alternatively sampled soil can be placed in a heavy-duty sieve (20 mesh/cm or near) and wash silt off with running water before immersing in a saturated salt bath (rubber gloves recommended). Grey false wireworm larvae can be counted as they float in the salt bath.

Grey false wireworm adults generally reinfest within the same area in the following season. Potential larval damage to seedlings in the following autumn may be estimated by sampling the ground in a similar manner in the spring. This is the time when the larvae are fully-grown and about to pupate or are pupating close to the surface. The observed density in spring is only indicative of populations in the next autumn as many variables (eg. soil management, climatic conditions, predators) can effect changes in the population size. In general a high population of larvae or pupae in spring would signal a strong chance of a high larval population density occurring during the next autumn.

Chemical control

Control with insecticides is recommended if the larval numbers exceed threshold levels. Soil incorporation of a registered organophosphate insecticide gives effective results in controlling crop damage. However, the broad scale application of insecticides can adversely affect non-target organisms, potentially killing predators and severely impacting on biodiversity. Insecticide seed dressings offer a more targeted and environmentally preferred option that can be applied before sowing for the control of damage by grey false wireworm larvae. Seed dressings generally are more expensive than the broad spread insecticides. Seed dressing insecticides are absorbed into the seedling and thus the application rates are much less than the broad scale applied insecticides. Seed dressings provide the chemical protection at the plant-insect interface and have little or no effect on natural enemies that live in the soil.

Post-sowing compaction

Post-sowing compaction can improve seedling vigour. Compaction by tyre rollers often gives good results with fine textured soils (eg. grey cracking clays in the Wimmera). This increases the soil/seed and soil/primary root contact giving better conditions for the seed to germinate. Compaction also blocks off the cracks and pathways available for the grey false wireworm larvae to detect and damage the seedlings. Damage by other seedling pests such as redlegged earth mites and blue oat mites is also minimised by post-sowing compaction.

The decision to compact has to be made with care. Prevailing soil conditions could interfere with the effectiveness of post-sowing compaction. Compacting with tyre rollers is not advisable under wet and sticky soil conditions.

Stubble management

Burying or elimination of fallen stubble before egg laying in early summer will increase adult mortality. The mortality is caused by physical injury and heat exhaustion resulting from the removal of shelter for the adults.

Use of higher seeding rate

The use of a higher seeding rate may be used as ‘insurance’ to overcome the risk of seedling and crop loss particularly in areas prone to grey false wireworm attack. This method of management can ensure sufficient seedling survival after insect attack. The ability of canola plants to compensate for reduced plant populations by developing more shoots is a major beneficial factor in the management of this pest. However, the use of a high seeding rate combined with insecticidal seed dressings will add to the initial establishment costs of canola.

Crop rotations

Crop rotations will influence the population levels of grey false wireworm. Compared to field pea, chickpea has been shown to increase the grey false wireworm numbers, therefore additional precautions should be considered in sowing canola in the first and second year after chickpea, as the effects of the increased larval pressure could persist for more than one year. If canola is to be sown, then grey false wireworm larval numbers should be checked in autumn prior to sowing.

Summary

Grey false wireworms are a significant pest of canola seedlings in Victoria, particularly in cracking clay soils. The larvae emerge during February and March and start feeding on organic matter in soil. The larvae ringbark the roots and hypocotyl causing mortality of canola seedlings but they do not damage seedlings of cereals or pulses. The larvae continue to survive on soil-organic matter and multiply in those paddocks. When the damage is noticed in the field it is too late to correct.

It is important to know the size of the larval population prior to sowing canola in order to reduce control costs. We recommend an integrated pest management (IPM) approach with cautious use of insecticides to control this pest. Tactics to consider include burying stubble and removing the shelters for adult beetles, monitoring the larval density before sowing, use of insecticide seed dressings or judicious use of soil insecticide, careful soil compaction after sowing and using a higher seeding rate. Management decisions need to be made well before sowing time. Combining more than one management technique will give best results in reducing damage to canola crops.

Further reading

McDonald G (1995) Wireworms and false wireworms in field crops. Agriculture Notes. Department of Primary Industries. AG0411. 3 pages.

Miles M (1997) Identification of false wireworms and other soil swelling pests of canola. Information leaflet. Department of Primary Industries. Horsham. Victoria. 2 pages.