The issue of pesticide resistance is linked to pesticide reduction through the fact that insecticide and herbicide resistance in insects or weeds commonly correlate with higher pesticide usage and higher control costs.

The many discussions of Bt resistance management plans (i.e. high toxin production in the Bt plants and the percentage of non-Bt plants grown in refuge fields) by public researchers and NGO persons generally neglect the vast knowledge and practical experience of resistance management strategies gained from sprayed insecticides, fungicides and herbicides. It is a fact that until now, with pesticides, there have been many more resistance management failures than successes.

The risk factors highly influencing the selection of herbicide resistant weeds are known and published: no crop rotation, no ploughing, only chemical weed control, the herbicides used with only one biochemical mode of action. Therefore a combination of "three Rs" is recommended to prevent weeds becoming herbicide resistant or to solve herbicide resistance problems. The rotation of crops, rotation of cultural practices (ploughing included) and a rotation of the modes of action of the used herbicides are advised.

Information on resistant weeds: new glyphosate resistant weeds

There is a very informative and freely accessible documentation of herbicide resistant weeds worldwide (www.weedscience.com). It seems this databank is neither widely known nor used by NGOs. Namely, two more cases of glyphosate resistant weed species, proven and published in 1999, have gone undetected: a glyphosate resistant rigid ryegrass (Lolium rigidum) in California/USA and a glyphosate resistant goosegrass (Eleusine indica) in Malaysia (at the homepage of weedscience.com please select "mode of action", then choose "glycines" and you will find the examples). In all likelyhood these two resistant weeds have been selected in no-tillage field systems.

Genetically modified crops are grown for resistance management purposes!

The databank presents a summary table of herbicide resistant weeds worldwide. In this summary 58 weed species and biotypes resistant to ALS inhibitor herbicides (such as imidazolinones, sulfonylureas, triazolopyrimidines) in 16 countries have climbed to position two, next to 61 triazine-resistant weeds in 22 countries; in November 1999 they took the lead. ALS inhibitor resistant weeds are most problematic in cereal, corn/soybean (!!), and rice production. It is likely that they will present farmers with greater problems in the next five years than triazine-resistant weeds have caused in the past 25 years.

By analysing and compiling databank information on the first proven occurence of weeds in soybean fields being resistant to ALS inhibitor herbicides in US states (Arkansas, Iowa, Minnesota, Ohio and others), I am able to conclude that these resistant biotypes should cause substantial problems for weed control. Consequently it follows that, in the USA, Roundup Ready soybeans are introduced and planted for weed resistance management purposes. Farmers switch from the overused herbicidal mode of action B (ALS inhibitor herbicides which resulted in resistant weeds within a few years) to the herbicidal mode of action G which is glyphosate, a chemical whose resistance risk is comparatively moderate.

But by doing only one rotational step, i.e. replacing imazethapyr, imazaquin, imazapyr, chlorimuron-ethyl and other ALS inhibitor herbicides in soybean with glyphosate sprayed on to genetically modified Roundup Ready soybean varieties, US farmers will hardly solve the problem. They will indeed spray their acres quickly into the next weed resistance catastrophy. Glyphosate resistant weeds will be selected easily from the increased glyphosate usage as many fields with a no-tillage history already have often a long field history of glyphosate usage. In the least, to my opinion, glyphosate resistant weeds will develop because of out-crossing transgenes.

It can be shown that in the USA and in China genetically engineered Bt-cotton is also introduced and "needed" for resistance management purposes.

Conclusions:

• The link between resistance problems and the newly grown genetically modified crops should be adressed seriously and properly.
• The US agriculture is no model! To quote experts of the herbicide industry: "From experience, we can conclude that rotation of herbicides alone is not enough to prevent the development of resistance." The USA should introduce integrated weed resistance management practices immediately!
• All interest groups, NGOs included, should become involved in acquiring knowledge on resistance management and resistance prevention because resistance problems, IPM measures, the quantity of pesticide usage and pesticide dependency are closely interlinked!

A more detailed version of these arguments (with six tables and reference literature) is available on request from the author or the PAN Germany bureau!

Crescentia Freudling
Freudlingc@aol.com

PAN Germany
pan-germany@t-online.de
Fax: +49-40-3907520

** NOTICE: In accordance with Title 17 U.S.C. Section 107, this material is distributed for research and educational purposes only. **