Persistent pests; aphids, mites, millipedes and earwigs

Author: | Date: 05 Feb 2014

Paul Umina,

Take home messages

  • Changes to insect complexes are presenting new pest challenges to farmers, particularly during the critical crop establishment period.
  • Growers are likely to face significant challenges in the future due to insecticide resistance in redlegged earth mites, green peach aphids and other crop pests.
  • Many decisions to manage crop establishment pests should be made well before sowing when there are far more control options available.

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 basic biology determined so effective control strategies can be devised. There is a considerable amount of information known about several pests, such as the redlegged earth mite (RLEM) and green peach aphid (GPA). For other crop pests, such as European earwigs and black Portuguese millipedes, there is little known and few management options are available.

Redlegged earth mites and insecticide resistance

RLEM (Halotydeus destructor) is a major pest species, 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, in 2006 chemical resistance was discovered in RLEM populations in Western Australia. Extremely high levels of resistance to several synthetic pyrethroids (> 200,000 fold in the case of bifenthrin) were detected using laboratory bioassays, and this has translated to significant yield losses in the field.

This resistance has been shown to have a genetic basis, persisting among mite populations after several generations of culturing away from the paddock. This means it can be passed on to offspring and will persist in the field indefinitely. Further surveys of RLEM have found this resistance to be more widespread than first thought. Resistance was tested from 115 paddocks across 85 properties in WA between 2007-2010. Twenty-eight individual paddocks were found to contain mites with resistance to the synthetic pyrethroid insecticides. These paddocks were spread across 19 separate properties. Further properties with insecticide resistance have been detected since 2010, although at this stage, resistance has not been detected outside of WA. Experts predict resistance in RLEM will spread to other states, including to Victoria.

Concerns surrounding other crop establishment pests and chemical use also exist. High levels of tolerance to several organophosphates and/or synthetic pyrethroids have been found in blue oat mites, Balaustium mites and Bryobia mites. This shows that current pesticide usage is unlikely to be a sustainable practice and also helps explain the increasing number of reports of these species persisting in the field after multiple chemical applications. Smarter chemical use is critical and a more strategic and integrated approach to pest management is needed. Table 1 provides some recommended management strategies for earth mites.

Table 1. Recommended control strategies for earth mites

Pre-season (previous spring / summer)

Assess risk

High risk when:

  • History of high mite pressure
  • Pasture going into crop
  • Susceptible crop being planted (e.g. canola, pasture, lucerne)
  • Seasonal forecast is for dry or cool, wet conditions that slow crop growth

 

If risk is high:

  • Ensure accurate identification of species
  • Use Timerite® (redlegged earth mites only)
  • Heavily graze pastures in early-mid spring

Pre-sowing

If high risk:

  • Use an insecticide seed dressing on susceptible crops
  • Plan to monitor more frequently until crop established
  • Use higher sowing rate to compensate for seedling loss
  • Consider scheduling a post-emergent insecticide treatment

 

If low risk:

  • Avoid insecticide seed dressings (esp. cereal and pulse crops) and plan to monitor until crop establishment

Emergence
  • Monitor susceptible crops through to establishment using direct visual searches 
  • Be aware of edge effects; mites move in from weeds around paddock edges

 

If spraying:

  • Ensure accurate identification of species before deciding on chemical
  • Consider border sprays
  • Spray prior to the production of winter eggs to suppress populations and reduce risk in the following season
  • Follow threshold guidelines

Crop establishment

As the crop grows, it becomes less susceptible unless growth is slowed by dry or cool, wet conditions

Green peach aphids and insecticide resistance

Aphids cause damage to crop plants from their feeding activities, as well as from the viruses they transmit. The green peach aphid (GPA – Myzus persicae) is an important pest of canola and several pulse crops, and vectors a number of important viruses such as cucumber mosaic virus, bean yellow mosaic virus and beet western yellows virus. Recent research has uncovered widespread resistance among GPA populations to several chemical classes in Victoria, South Australia, New South Wales, Queensland and Western Australia.

More than 40 populations (over an area spanning more than 1700 kilometres across eastern Australia and 800 kilometres in WA) have been collected and tested for resistance. Almost 70% of all populations showed high resistance to synthetic pyrethroids (e.g. alpha-cypermethrin), indicating resistance to this chemical group has become significantly more common over the past 5-10 years.

Most alarming was the discovery that about 50% of populations were found to be resistant to pirimicarb (e.g. Pirimor®). The confirmation of widespread resistance to pirimicarb is particularly concerning for pulse and oilseed growers because this chemical has been a fallback for aphid populations resistant to other chemical groups.

Pirimicarb is aphid-specific and less harmful to other invertebrates when applied to crops, so is compatible with an integrated pest management (IPM) approach. In 2010, the first documented case of resistance to pirimicarb in Australia was identified in a GPA population from WA. This was the first confirmed instance of GPA resistance to carbamates in Australia, although tolerance to pirimicarb was identified many years previously by state agricultural entomologists in WA and in other states.

The recent survey also showed resistance to organophosphate chemicals (e.g. dimethoate) is widespread across Australia; something that has been demonstrated in previous research surveys.

Monitoring aphid populations and reducing ‘insurance sprays’ will help to prolong the life span of insecticides used to control GPA. It is recommended that growers reduce the availability of alternate GPA hosts between growing seasons by controlling summer and autumn weeds, particularly wild radish, wild turnip, capeweed and volunteer canola and lupins. A border spray in autumn/early winter, when aphids begin to move into crops, may provide sufficient control without the need to spray the entire paddock. Table 2 provides some recommended management strategies for green peach aphids.

Table 2. Recommended control strategies for green peach aphids

Summer / autumn

Assess risk (virus)

High risk where

  • Summer rainfall creates a Brassica green bridge
  • Warm conditions favour early aphid build-up and timing of flights

If high risk:

  • Use an insecticide seed treatment to manage virus spread (e.g. BWYV) by green peach aphid

 

Manage Brassica weeds and volunteers (ideally area wide) 3-4 weeks before sowing

Sow early to promote early flowering in spring before aphids peak

Winter

Monitor crops for aphid colonisation from late winter when daily temperatures start to rise.

 

High risk when:

  • Mild winter
  • Aphids forming dense colonies on growing tips
  • Forecast is for warm and dry conditions that favour aphid development
  • No beneficial activity and/or aphid parasitism

 

If high risk:

  • Consider border sprays with a selective aphicide (pirimicarb) to prevent/delay build-up and retain beneficials

Spring

Monitor trends in aphid and beneficial populations in crops over time. Use thresholds to guide spray decisions, considering crop stage and moisture stress.

 

High risk when:

  • Infestation rapidly increasing during early flowering to bud formation
  • Forecast is for warm and dry conditions to continue
  • Low/no parasitism and beneficial activity (note: this can also happen if broad-spectrum insecticides are used to control native budworm and diamondback moth)

 

If spraying:

  • Use soft products (pirimicarb or petroleum spray oils) to retain beneficials
  • Rotate insecticide MOAs to reduce resistance selection in green peach aphid.

European earwigs

There are many species of earwigs in Australia. Some are beneficial while others, particularly the European earwig (Forficula auricularia), are increasing in status as agricultural crop pests.

The paddock habitat for earwigs and other insects has altered in recent years with on-farm practice change. Retained crop residues on the soil surface are thought to contribute to populations building up and damaging crops during autumn and early winter. Increases in earwig populations have also been linked to increases in soil organic matter.

European earwigs mainly attack canola but will also attack cereals, lupins and some legume crops. Damage can be scattered because of their patchy distribution. Earwigs chew the stems and cotyledons of emerging seedlings, killing plants or slowing plant development. As the plant grows, foliar damage includes shredded leaf tips and jagged holes in leaves. Earwigs can completely defoliate young seedlings leaving only stems or bare ground in patches. They can also chew through seedpods.

Earwigs feed together at night, and in many cases, damage will start along the edges of a paddock. Earwig damage to plant leaves closely resembles feeding damage caused by slugs.

Control options in broadacre crops are limited. Cultural control practices such as reducing stubble retention and decreasing available refuges are likely to be the most effective strategy for managing populations over time. Burning has been successful in reducing populations in some instances. If any damaged areas need to be reseeded, a higher seeding rate is recommended to compensate for further damage.

There are no sprays registered for in-crop control of earwigs. In some states, fipronil seed dressings are registered for protection of sorghum and sunflower crop seedlings from black field earwigs, while imidacloprid seed dressings are registered for protection of maize, sorghum, sunflower and sweetcorn from black field earwigs.

Black Portuguese millipedes

In the past five to 10 years, damage caused to some broadacre crops by black Portuguese millipedes (Ommatoiulus moreleti) has been increasing. Similar to earwigs, the increase has been linked to stubble retention, no-till farming practices and improvements in soil organic matter, which have provided a more favourable habitat for millipedes to survive and reproduce. Recent wet summers have contributed to a population build-up in some parts of southern Australia while planting of more vulnerable crops, such as canola, has led to increased damage.

Millipedes feed on leaf litter, damp and decaying wood, fungus and vegetable matter like tender roots, mosses, pollen or green leaves on the ground. They can play a role breaking down organic matter in the soil. As a result, they occur in greater numbers in undisturbed leaf litter and organic mulch and in areas where winter weeds form a mostly continuous ground cover. Millipedes are generally not as numerous in cultivated areas or bare ground.

Since black Portuguese millipedes generally feed on organic matter, crop feeding damage is relatively rare. Black Portuguese millipedes occasionally attack living plants by chewing the leaves and stems. It has been suggested that millipedes feed on crop plants when they are seeking moisture but this has not been confirmed.

Most reported millipede damage has occurred in emerging canola crops on black organic soils with heavy stubble loads, although damage has also been observed on lighter soils. In canola, millipedes remove irregular sections from the leaves and can kill whole plants if damage is severe. Damage to cereals can also occur where the stems of young plants are chewed.

The presence of black Portuguese millipedes does not always result in damage. There have been many instances where no damage has occurred despite large millipede populations. Millipedes are mostly active and feed at night, which is the best time to check if they are causing damage to canola plants.

There are no insecticides registered to control millipedes in broadacre crops and control options are limited, although there are some measures that will reduce population sizes. Reducing the amount of trash and stubble over summer and early autumn is likely to be the most effective way to reduce millipede numbers. Burning stubbles may reduce millipede populations. Early sowing of high-vigour varieties at a higher seeding rate will help compensate for seedling losses from feeding damage.

Acknowledgements

Kym Perry, SARDI Entomology

Garry McDonald, cesar

Melina Miles, QLD DAFF

Owain Edwards, CSIRO

Contact details

Paul Umina

03 9349 4723

pumina@cesaraustralia.com

GRDC Project Code: CSE00046, UM00049, CES00001,