Willow weed
NZ populations of willow weed
Persicaria persicaria developed
herbicide resistance in the
late 1970s.

Herbicide resistance in New Zealand

Development of herbicide resistance

Herbicide resistance is generally less common than fungicide or insecticide resistance. Most weed species complete their life cycle only once each year, whereas fungi and insects often complete many life cycles within one year. This allows resistance genes to build up much more quickly within fungal or insect populations than with weeds.

Following applications of insecticides and fungicides, most susceptible individuals within a sprayed area are killed, leaving only resistant individuals to multiply. In contrast, because herbicide applications do not kill dormant weed seeds, weeds which lack resistance genes are difficult to remove from the population - thereby delaying the onset of resistance.

However, resistance can eventually build up if the same herbicides are applied to a weed population many times in succession. Herbicide resistance has developed to triazine herbicides such as simazine and atrazine in many countries. This is because the persistence of these residual herbicides allows them to continue killing susceptible individuals, as dormant weed seeds germinate, for many months after application. If these herbicides have been applied for year after year, such as in maize crops, orchards and nurseries, resistance has finally built up. Although many cases have now been reported, it is still relatively uncommon.

New Zealand cases

NZ populations of fathen and willow weed developed resistance to triazines such as atrazine, terbuthylazine and cyanazine in maize fields in the late 1970s. Initially this resistance was detected in Waikato maize, but is now more widespread. Other cases of triazine resistance have developed more recently, such as black nightshade resistant to terbuthylazine and cyanazine in pea crops in Manawatu.

Until the 1980's, it was thought herbicide resistance was likely to develop only with residual compounds such as the triazines because of the prolonged selection pressure they apply to germinating seeds. Many people believed it would not be a problem with knockdown herbicides such as paraquat and 2,4-D, because they have no effect on seedlings which germinate after the herbicide has been applied.

However, in New Zealand, resistance to MCPA and 2,4-D was discovered in nodding thistle, slender winged thistle (Carduus pycnocephalus) and giant buttercup in the 1980s. Resistance to dalapon has also been found in Chilean needle grass biotypes.

In USA and elsewhere in the world, resistance has developed quite rapidly to a group of herbicides known as the sulfonylureas. So far in New Zealand, the only reported case of resistance to sulfonylureas has been to chlorsulfuron in chickweed biotypes in Southland cereal crops.

More information on herbicide resistance elsewhere in the world can be found at www.weedscience.com.

NZ Herbicide Resistance Task Force

The New Zealand Herbicide Resistance Task Force (NZHRTF) is a group of NZ Plant Protection Society members who are actively involved with researching herbicide resistance within New Zealand. They include scientists from Crown Research Institutes, universities and chemical companies. Collectively they collate information on cases of herbicide resistance within New Zealand and formulate strategies for managing and avoiding herbicide resistance problems.

Current members of the NZHRTF are: