Modelling binary mixtures of herbicides in populations resistant to one of the components: evaluation for resistance management.

BACKGROUND: Herbicide mixtures are commonly proposed to delay the selection of herbicide resistance in susceptible populations (called the SM strategy). However, in practice, herbicide mixtures are often used when resistance to one of the two active ingredients has already been detected in the targeted population (called the RM strategy). It is doubtful whether such a practice can select against resistance, as the corresponding selection pressure is still exerted. As a consequence, the effect of mixtures on the evolution of an already detected resistance to one of the herbicides in the combination remains largely unexplored. In the present work, a simple model was developed to explore further the necessary and sufficient conditions under which a binary RM strategy might stabilise or even reduce resistance frequency. RESULTS: Covering the hypothetical largest range of parameters, 39% of 9000 random simulations attest that the RM strategy might theoretically reduce resistance frequency. When strong enough, high genetic cost of resistance, negative cross-resistance between the herbicides associated in the mixture and reduced selection differential between resistant and susceptible plants can counterbalance the resistance advantage to one of the two applied herbicides. However, the required conditions for an RM strategy to ensure resistance containment in natural conditions seldom overlap with experimental parameter estimates given in the literature. CONCLUSION: It is concluded that the sufficient conditions for an RM strategy to be effective would rarely be encountered. As a consequence, the strategy of formulating mixtures with herbicides for which resistance has already been detected should be avoided.

Dominance variation across six herbicides of the Arabidopsis thaliana csr1-1 and csr1-2 resistance alleles.

Dominance of a resistance trait can be defined as a measure of the relative position of the phenotype of the heterozygote RS compared with the phenotype of the two corresponding homozygotes, SS and RR. This parameter has been shown to have primary importance in the dynamics of pesticide resistance evolution. Literature on insecticide resistance suggests that dominance levels in the presence of insecticide vary greatly from completely recessive to completely dominant. With insecticides, both the chemical applied and the dosages used have been demonstrated to affect the dominance. By contrast, almost all herbicide resistances have been found to be inherited as partially to totally dominant traits. This discrepancy between weeds and insects may partly result from the methodologies applied to measure the dominance, ie a single dose for herbicide versus several doses for insecticide. Using two well-known resistances (csr1-1 and csr1-2) to acetolactate synthase (ALS) inhibitors in Arabidopsis thaliana (L) Heynh (mouse-ear cress), we used several herbicide doses to determine the dominance level to six ALS-inhibiting herbicides. The dominance level in the presence of herbicide varied from completely dominant to completely recessive, depending on the resistance allele and the herbicide tested. The dominance of the csr1-1 and csr1-2 resistance alleles ranged from 0 (completely recessive) to 1.1 (dominant) and from 0 to 0.3 (partially dominant), respectively. The recessivity of some resistance alleles in the presence of herbicide could lead to the development of improved resistance management in order to delay or avoid herbicide resistance evolution, especially in the control of outcrossing weed species.

The dominance of the herbicide resistance cost in several Arabidopsis thaliana mutant lines.

Resistance evolution depends upon the balance between advantage and disadvantage (cost) conferred in treated and untreated areas. By analyzing morphological characters and simple fitness components, the cost associated with each of eight herbicide resistance alleles (acetolactate synthase, cellulose synthase, and auxin-induced target genes) was studied in the model plant Arabidopsis thaliana. The use of allele-specific PCR to discriminate between heterozygous and homozygous plants was used to provide insights into the dominance of the resistance cost, a parameter rarely described. Morphological characters appear more sensitive than fitness (seed production) because 6 vs. 4 differences between resistant and sensitive homozygous plants were detected, respectively. Dominance levels for the fitness cost ranged from recessivity (csr1-1, ixr1-2, and axr1-3) to dominance (axr2-1) to underdominance (aux1-7). Furthermore, the dominance level of the herbicide resistance trait did not predict the dominance level of the cost of resistance. The relationship of our results to theoretical predictions of dominance and the consequences of fitness cost and its dominance in resistance management are discussed.

PCR-based detection of resistance to acetyl-CoA carboxylase-inhibiting herbicides in black-grass (Alopecurus myosuroides Huds) and ryegrass (Lolium rigidum gaud).

A simple method based upon allele-specific PCR was developed to detect an isoleucine-leucine substitution in the gene encoding chloroplastic acetyl-coenzyme A carboxylase (ACCase) in two gramineous weeds: Lolium rigidum Gaud and Alopecurus myosuroides Huds. Analysis of 1800 A myosuroides and 750 L rigidum seedlings showed that the presence of ACCase leucine allele(s) conferred cross-resistance to the cyclohexanedione herbicide cycloxydim and to the aryloxyphenoxypropionate herbicides fenoxaprop-P-ethyl and diclofop-methyl. Seedlings containing ACCase leucine allele(s) could be either sensitive or resistant to the aryloxyphenoxypropionate herbicides haloxyfop-P-methyl and clodinafop-propargyl. Successful detection of resistant plants in a field population of A myosuroides was achieved using this PCR assay. Using it with basic molecular biology laboratory equipment, the presence of resistant leucine ACCase allele(s) can be detected within one working day.