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RESISTANCE MANAGEMENT IN AUSTRALIAN COTTON Neil W. Forrester, NSW Agriculture

&

Fisheries, Agricultural

Research Station, Narrabri, NSW. 2390

The Australian Field Crops Resistance Management Strategy has been in place now for

7

seasons.

The

impact of the Strategy on resistance levels

is

monitored in

three

areas; the Namoi and Gwydir river valleys of nonhem NSW (a temperate cotton monoculture), the Emerald Irrigation Area of central Queensland (a sub tropical mixed cropping area) and a sample of the unsprayed Refugia centred on Inverell

in

nonhern NSW. Resistance to both pyrethroids and endosulfan is assessed, as these two groups account

for

over 80% of the insecticides used

against Heliothis

spp. in Australian cotton. Studies with the oxidase inhibiting synergist piperonyl butoxide (Pbo) have indicated that the dominant mechanism ofpyrethroid resistance in the field is via oxidative detoxification. This has led to

the

investigation of two possible chemical countermeasures for managing pyrethroid resistance: synergists and structurally modified pyrethroids (also called resistance breaking pyrethroids). A brief summary of each of these findings will be given followed by an outline of the future for managing pyrethroid resistance.

PYRETHROID RESISTANCE

Pyrethroid resistance levels have continued to increase slowly. The reduction of the pyrethroid window from 6 to 5 weeks has had a clear beneficial impact. The double selection which was occurring in the 6th week of Stage 2 has now been avoided and in fact, for the first time since the introduction of the Strategy, resistance levels dropped immediately after Stage 2. Normally they continue to increase and

peak

at a very much higher level in the 2nd -

4th

week of Stage 3.

However, after this initial decrease, they rose again later in Stage 3 due to the emergence of the survivors from larval selection in Stage 2. This resulted

in

a "twin peak" in both cotton study areas. The reduction in the window has slowed the increase

in

pyrethroid resistance, but it will not stop it. Of major concern is the very rapid rise in pyrethroid resistance within the window, experienced in the past two seasons. Resistance levels can change from satisfactory (about 25%) to

uncomfortably high (70%) within 5 weeks. This is the critical period for growers

and consultants as the pyrethroids are being used and high

armigera

pressure at this

time, will result in poor field control. We must keep the pyrethroid window at 5

weeks and

try

to break the rapid adult moth selection cycle.

362

ENDOSULFAN RESISTANCE

The resistance levels at the start of the season were encouragingly low.

However, levels had risen quite markedly towards the end of the season,

particularly at Emerald where endosulfan use is higher. However, we have seen high levels in the past, and hopefully, they may return to low levels again by the start of the season. However, it is quite clear that we have incipient endosulfan resistance and that any increased endosulfan selection pressure on Heliothis armigera will probably tip the balance. We must retain the efficacy of endosulfan and avoid a return to the endosulfan resistance problems of the mid 70's. We need to keep the pyrethroids working in Stage 2, to avoid the increased use of

endosulfan at this time of increasing

armigera

pressure.

PBO

Piperonyl butoxide has continued to give high levels of suppression of field pyrethroid resistance in all areas, last season averaging about 75% suppression during the critical Stage 2 pyrethroid window. Pbo is not effective on the intractable nerve insensitivity resistance mechanism and this is seen as the low level of

residual resistance (usually less than 10%) left after adding Pho to pyrethroids.

Research has continued on developing light stable formulations of Pbo but so far, no improvement has been made over the currently available conunercial

formulations. Residual activity at 8:1 (Pbo:pyrethroid) is about I day while increasing the rate of Pbo (15: 1) can push it out to perhaps 2 days. It would also help to apply Pbo in the early evening, to allow maximum exposure to the nocturnal moths before any degredation by sunlight.

RESISTANCE BREAKING PYRETHROIDS

A pyrethroid, which was shown to overcome metabolic resistance in a laboratory strain of resistant larvae, was tested last season on field collected

material. It was shown to be as effective as the pyrethroid

I

Pho mix on both moths and larvae. To be fully effective, a Resistance Breaking Pyrethroid needs to satisfy the following 5 requirements. It must :-

•resist metabolic detoxification by resistant Heliothis amzigera

• be light stable

• have low manunalian toxicity

• be a relatively active compound (effective at or less than 20-80 g ai

I

ha )

•must have persistence (i.e. low volatility)

We do not yet have a pyrethroid which meets all 5 criteria but I believe it is only a

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363

matter of time before we do. Commercialisation of that molecule will then be the major obstacle.

FUTURE

In the longer term, we can probably look forward to new effective chemical groups, improved biological pathogens and genetically engineered host plant resistance. In the meantime, we must make do with our currently available limited resources. We must utilise the full range of control techniques. These are:-

1) Chemical - continue to follow the rotation Strategy

- use mixtures of ovicides when egg pressures warrant - target sprays to egg hatch

- avoid pyrethroids during high

armigera

pressure

- add Pbo to the second pyrethroid spray to disrupt the rapid adult selection cycle

- use mixtures of bacterial pathogens (eg.

Bacillus thuringiensis)

with conventional insecticides

2) Biological - use soft chemicals early (eg. endosulfan, thiodicarb and B .t.) to preserve the maximum benefit from early season biological control agents. This also prevents flaring of secondary pests such as mites and whitefly.

3) Cultural

- utilise host plant resistance wherever possible (eg.okra leaf varieties off er some degree of control, particularly for mites).

- grow early maturing crops to avoid late season

armigera

problems (similarly, new investments in cotton

production should ideally be located in the western areas of the cotton belt where

armigera

is much less of a problem).

- avoid unfavourable cropping patterns conducive to a buildup of

armigera (

eg. December flowering corn and sunflowers in predominantly cotton areas) - cotton crop residues should be thoroughly

cultivated to minimise survival of oveIWintering pupae

The key to managing resistance in

Heliothis armigera

in the furure will be to vary the control practices so that the selection pressure will be insufficient for

armigera

to develop resistance to any one control measure. This should be the basis for long term, sustainable, economic pest control in cotton.

- -

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70

50

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1988/89 1989/90

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