Susceptibility variation to new and established herbicides: Examples of inter-population sensitivity of grass weeds

The objectives of this study were to describe the intra-specific variation in herbicide response of weed populations when subjected to new vs. well-established herbicides, and to assess distributions of logLD50- and logGR50-estimates as a potential indicator for early resistance detection. Seeds of two grass weeds (Alopecurus myosuroides, Apera spica-venti) were collected in southern Sweden, mainly in 2002. In line with the objectives of the study, the collections sites were not chosen for noted herbicide failures nor for detected herbicide resistance, but solely for the presence of the target species. For each species, seedlings were subjected to two herbicides in dose-response experiments in a greenhouse. One herbicide per species was recently introduced and the other had been on the market for control of the species for a decade, with several reports of resistance in the literature. Fresh weight of plants and a visual vigour score were used to estimate GR50 and LD50, respectively. Resistance to fenoxaprop-P-ethyl in A. myosuroides was indicated by the LD50-estimates to be present in frequencies sufficient to affect the population-level response in 9 of 29 samples, and was correlated to response to flupyrsulfuron, while low susceptibility to isoproturon in A. spica-venti populations was not linked to the response to sulfosulfuron. In the study as a whole, the magnitude of the estimated herbicide susceptibility ranges differed irrespective of previous exposure. No consistent differences were found in the distribution of LD50-estimates for new and “old” herbicides, and normality in the distribution of estimates could not be assumed for a non-exposed sample, even in the absence of an indication of cross-resistance.

Research highlights
► Slowly evolving pesticide resistance is difficult to detect and monitor.
► Inter-populations differences in LD50- and GR50-estimates from dose-response experiments was substantial.
► Neither logLD50- nor logGR50-estimates were normally distributed.
► No indication that logLD50- or logGR50-estimates were less skewed for a new herbicide compared with an old.
► Differential growth rate explained population differences in susceptibility