Ground Cover North : Ground Cover 073 March-April 2008 - North
MARCH -- APRIL 2008 GROUND COVER Crop protection Alarm grows over 'unkillable' mite Already costing the industry half a billion dollars a year, the effects of redlegged earth mites could escalate if resistance becomes widespread BY CATHERINE NORWOOD n The resistance of redlegged earth mites to chemical controls is, so far, an isolated problem, identified at only a few locations in Western Australia. But it is a problem researchers want to get on top of as quickly as possible. CSIRO estimates that redlegged earth mites already cost the agricultural industry more than $500 million a year, making them one of the industry's most significant pests. Widespread resistance could see the impact of these tiny pests escalate. Redlegged earth mites are a common problem for many pulse and oilseed crops, and pastures. Canola is particularly susceptible: untreated seedlings can be wiped out as the mites attack germinating plants, making them weak and unproductive, or killing them outright. Cereals are less susceptible. There are two common strategies for redlegged earth mite control. One involves killing the earth mites in autumn to protect emerging crops. The other involves killing adult earth mites in spring to prevent the production of eggs, which lie dormant during summer to hatch in the following autumn when favourable conditions prevail. Both strategies rely heavily on chemical controls. Department of Agriculture and Food, WA (DAFWA), senior entomologist Geoff Strickland says over-reliance and overuse of a single chemical is a major contributor to the evolution of resistance. "Resistance is genetically based, so if you spray and it doesn't seem to work, and you spray again at a higher rate, what you effectively do is select for the most resistant strain, and they get harder and harder to kill," Mr Strickland says. "If a chemical isn't working, avoid reapplying the same chemical; it will only exacerbate the problem." Synthetic pyrethroid products in particular have become progressively cheaper over the past two decades, and this has made them a popular pesticide choice for earth mite control. This is also the chemical class to which redlegged earth mites are showing resistance. Mr Strickland says samples of earth mites were collected from two sites at which growers had reported chemical failures, one near Esperance and one near Cranbrook. "The really alarming thing was that the resistance levels were monumentally high -- these mites were virtually unkillable," Mr Strickland says. "Our gut feeling is that the problem is not restricted to these two farms, and we're developing a project now to help identify the extent of the problem, as well as strategies to counter it." The discovery of chemical resistance in redlegged earth mites was made by researchers at the University of Melbourne's Centre for Environmental Stress and Adaptation Research (CESAR). CESAR has been researching earth mites for more than a decade. Research and extension officer at CESAR Dr Paul Umina says mites from the WA sites were bred through several generations to establish that the resistance was being passed on to each generation. "The very high level of resistance is worrying, particularly as the mites were resistant to a number of different chemicals, with different active ingredients, although they were all synthetic pyrethroids," Dr Umina says. He says there is no evidence of resistance to organophosphates, also commonly used to control redlegged earth mites, and the development of widespread resistance is by no means inevitable. "Understanding the nature of field resistance in redlegged earth mites is critical in order to devise sound pest-management recommendations that minimise the risk of resistance development," he says. Back in WA, DAFWA is setting up an early identification program, including a simple laboratory test, which will provide an indication of earth mite resistance within three to four days. The test was developed by CESAR, and when the indicator test is positive, further earth mite samples will be collected and sent to its Melbourne laboratory for a full toxicology evaluation. Mr Strickland says he is keen to hear from any growers who experience resistance by redlegged earth mites to their normal chemical control program as soon as it occurs. He suggests that new strategies will need to be developed to counter resistance. Top of the list is rotating chemicals to reduce the likelihood of resistance developing. Other chemical options may include early pasture freezing, with an application of a herbicide timed to kill pasture and weeds in spring to effectively starve the mites before they begin laying eggs. Less drastic is the use of CSIRO's Timerite® system, a web-based decision-making tool for the timing of chemical applications for pasture, also aimed at killing off adults in spring, before their dormant phase begins. Above all, non-chemical options will ultimately be the most important to control resistance. Mr Strickland suggests planting canola, pulses and legumes as early as possible, before cool weather conditions allow for redlegged earth mite hatching, which will give seedlings a better chance to establish themselves. Planting cereals following a pasture rotation could also help reduce the build-up of earth mite numbers as cereals are less susceptible to attack. Growers with stock are encouraged to heavily graze paddocks as part of a rotational or crash-grazing system, rather than applying a set stocking rate. Stock will literally eat the earth mites along with the pasture during intensive grazing. GRDC Research Code DAW00127 More information: Geoff Strickland, 08 9368 3756, firstname.lastname@example.org; Peter Mangano, 08 9368 3753, email@example.com; Svetlana Micic, 08 9892 8591, firstname.lastname@example.org; Paul Umina, 03 8344 2522, email@example.com 8 YEAR WARRANTY* Australia's longest agricultural tyre warranty. *Conditions apply Our unique 23° bar angle delivers optimum tyre footprint and traction, saving you time and fuel. Tyres with the right design for broadacre farming... Our Firestone All Traction 23° rear tractor tyre is the ideal tyre for broadacre farms. Designed to ensure your tractor travels along the top while tractors with 45° tyres dig in. So you save on fuel. You'll feel the improved traction. The flexible sidewall rubber compound and the super- tough tread rubber deliver a great ride and improved tyre wear. The Firestone All Traction 23° is the preferred tractor tyre for many Original Equipment Manufacturers, including John Deere, New-Holland, and Case. That's Firestone. Freecall 1800 788 688 for your local approved Firestone Farm Tyre Dealer www.tyres4u.com.au ATYR0014/GC Redlegged earth mites can cause extensive damage to emerging crops and pasture, particularly pulses and legumes. PHOTO: ANDREW WEEKS, CESAR REDLEGGED EARTH MITES nRedlegged ear th mites (Halotydeus destructor) are one millimetre in size and black, with oval shaped bodies and eight red-orange legs. nThey attack and suck fluids from emerging crop seedlings -- typical feeding signs are white blotches on the foliage. nDamage is prevalent under cool, moist conditions. nAfter over-summering, mites hatch once temperatures drop below 20˚C over six days, and where adequate moisture (about 15mm) is available. nRedlegged ear th mites grow to adults within four to six weeks. nHeavy infestations can rapidly kill emerging seedlings. nCropping paddocks should be inspected regularly for redlegged ear th mite populations before sowing and at emergence -- the best times for inspections are on cloudy days, in the early morning or late afternoon.
Ground Cover 072 January-February 2008 - North
Ground Cover 074 May-June 2008 - North