Crop insects and mites Resistance issues and integrated management

| Date: 14 Feb 2008

 Paul Umina, CESAR - Centre for Environmental Stress and Adaptation Research, The University of Melbourne

• Chemical resistance and secondary pests are emerging as genuine threats to the grains industry
• The key to successful IPM is accurate identification of pest and beneficial species
• Different mite species require different management strategies

Pest species within the grains industry pose a serious threat as farming practices change. To avoid costs associated with crop failure and increases in pesticide usage, potential pest species must be identified and their biology determined so effective control strategies can be devised. Over the last decade, a large amount of research has been carried out on a number of important pests, such as blue oat mites and the redlegged earth mite (RLEM). This has led to important breakthroughs in the way we now control these mites. For example, spring-spraying and seed treatments have been widely adopted for control of the RLEM in many regions. However, the recent evolution of chemical resistance in RLEM highlights the need for alternative control strategies and integrated management.

Underpinning an integrated pest management approach is correct identification and monitoring of both pest and beneficial insects. Misidentification of pests can cost growers money through ineffective control strategies and pesticide applications. Monitoring of pest and beneficial numbers is also critical for making informed control decisions.

Research at the Centre for Environmental Stress and Adaptation Research (CESAR) aims to develop improved integrated management strategies that focus on all pests within cropping and pasture systems and incorporate control options that minimize effects on key functional invertebrate groups.

Earth mites and chemical resistance

RLEM is a major invertebrate pest, particularly to establishing crops and pastures. Mite feeding significantly reduces seedling survival and development and will often lead to entire paddocks needing to be re-sown. For decades, RLEM have been controlled relatively effectively with broad-spectrum pesticides. However, researchers at CESAR have recently discovered chemical resistance in some RLEM populations. Extremely high levels of resistance to several synthetic pyrethroids - detected for the first time using laboratory bioassays - have led to significant yield losses in the field (Figure 1).

This resistance has been shown to have a genetic basis, persisting after several generations of culturing away from the paddock. This means it can be passed on to offspring and could persist in the field indefinitely. In total, resistance has now been demonstrated for five synthetic pyrethroids, all of which are currently registered to control RLEM in Australia. Further surveys of RLEM have also found this resistance to be more widespread than first thought, suggesting that it may be spreading and has potential to be a widespread problem. It is encouraging that resistance to organophosphate chemicals has not been detected, although there is evidence of genetic tolerance in some populations of RLEM.

Concerns surrounding other establishment pests and chemical use also exist. CESAR researchers have found high levels of tolerance to several organophosphates and/or synthetic pyrethroids in blue oat mites, the lucerne flea and in two emerging mite pests, Balaustium and Bryobia mites.

For example, in laboratory bioassays, estimates showed lucerne fleas to be 195 times more tolerant of alpha-cypermethrin, 57 times more tolerant of bifenthrin, 26 times more tolerant of methidathion, and 19 times more tolerant of omethoate compared with RLEM. Balaustium and Bryobia mites also have a high natural pesticide tolerance compared to RLEM and are proving to be more difficult to control in the field. For example, Balaustium mites were found to be 272 times more tolerant than RLEM for methidathion, 260 times more tolerant of bifenthrin and 129 times more tolerant to omethoate compared with RLEM. Bryobia mites exposed to methidathion were shown to have 64-fold greater tolerance compared with RLEM and an 11-fold greater tolerance to bifenthrin.

These findings show that current pesticide usage is unlikely to be a sustainable practice and may help to explain the increasing number of reports that these species are persisting in the field even after multiple chemical applications. Smarter chemical use is critical; synthetic pyrethroids should be avoided for the control of lucerne flea and Balaustium mites may be difficult to control with all currently registered pesticides.

Beneficial species (natural enemies)

Naturally occurring beneficial species play a vital biological control role in many cropping systems. Most species are highly mobile and will move from crop to crop if left unsprayed. They are able to help keep pest populations under control.

Beneficial classifications include:
• Predators: generalist; consume a wide range of prey; free living
• Parasites: specialised and target species; feed on or in the body its host
• Diseases: insect fungal, viral and bacterial infections

Common beneficial species likely to be encountered include predatory mites, lacewings, hoverflies, ladybird beetles, carabid beetles, damsel bugs, spiders and parasitic wasps.

Whilst there are organisations that breed beneficial insects and mites for release, the most effective strategy is likely to be the preservation of those already in the system. Other factors involved in supporting beneficial invertebrates in the system include alternate food sources (eg. nectar sources, non-pest hosts) and refuge habitat (eg. remnant vegetation, trap crops).

‘Softer’ chemicals

Although chemical control is still an important part of an integrated pest management strategy, there needs to be a shift from using broad-spectrum pesticides to more selective alternatives. Broad-spectrum chemicals invariably kill non-target organisms.

The use of more selective or ‘soft’ pesticides is an effective management tool that facilitates – rather than disrupts – the natural biological control that already exists. By specifically targeting plant-feeding invertebrates, they allow beneficial species to remain in the system to help suppress pest numbers.

Seed dressings may also be an alternative control option and will delay applications of foliar sprays giving beneficial insects time to build up. Seed dressings need some thought process on potential pest pressures prior to sowing as many different dressings are available. Seed germinating baits are a quick and effective monitoring method to assess potential soil inhabiting pests that attack seeds and seedlings.

A review of pest occurrences in 2007 and ‘PestFacts south-eastern’

‘PestFacts south-eastern’ is a free service designed to keep growers and advisers informed about pest-related issues – and solutions - as they emerge during the growing season. It is distributed as an electronic newsletter and aims to help growers achieve maximum yield and quality for the lowest cost by providing timely information about pest outbreaks, effective controls and information about relevant and new research findings. To provide this service PestFacts draws on the field observations of consultants, growers and industry specialists across south-eastern Australia as they report on the location and extent of invertebrate outbreaks. It also issues warnings (or reminders) for a range of invertebrate pests of all crops, including pulses, oilseeds, cereals and fodder crops.

PestFacts is part of the GRDC-funded National Invertebrate Pest Initiative (NIPI), designed to help advisers and growers deal with the increasing challenges being presented by mites and insects. It is based on the PestFax model that has run successfully in WA for many years.

The information generated by PestFacts can also be used to gain an idea of the occurrence and location of pest problems. This provides an opportunity for awareness, discussion and ongoing evaluation of changing pest importance. Table 1 shows the invertebrate pest reports received by PestFacts for Victoria and New South Wales in 2007.

PestFacts is widely distributed to over 700 recipients and the level of contributions will hopefully continue to expand to provide an increasing network of information. Developing the most effective strategies for the control of grains pests will result from the integration of a number of advances in our understanding of pest biology/ecology.

Further Reading

PestFacts south-eastern:

Resistance threatens to derail redlegged earth mite control
Farming Ahead (2007)184: 47-49 (Umina).

Contact details

Paul Umina Ph: 03 8344 2522

Andrew Weeks Ph: 03 8344 2521