Abstract
Genetically modified maize tolerant to broad-spectrum herbicides may greatly alter weed flora composition, abundance and therefore affect organisms of higher trophic levels, including herbivore and detritivore arthropods and their natural enemies. This three-year study measured the effects on arthropods of an intensive use of broad-spectrum herbicides in comparison with one application of conventional pre-emergence herbicide. Numbers of arthropods were measured by three techniques: visual counts on plants, catches in pitfall and yellow sticky traps. Weed density was much higher in conventional treatment in the first year, showed significant difference in the second year, but was no significant difference in the third year. Counts of arthropod taxa were significantly different only in the first year in the two kinds of weed management systems. In visual counts Cicadellidae and Aphididae among herbivores, the two main generalist predators, Orius spp. and Araneae, and the family Coccinellidae were more abundant on plants treated twice with glyphosate. In pitfall there were higher records in glyphosate-treated plots for Myriapoda but the opposite was seen for Carabidae counts. The yellow sticky traps catches were higher in the glyphosate-treated plots for Cicadellidae and Mymaridae, and lower for Thysanoptera. Most of the significant differences found between herbicide regimes disappeared when abundances of weeds (monocotyledons and dicotyledons) were introduced into the analysis as covariates; this finding signals weed abundance as the main cause of arthropod abundance alteration. However, only a drastic alteration of weed abundance causes significant changes in arthropod densities.
Highlights
Modified herbicide-tolerant (GMHT) maize was cultivated in millions of hectares in 2013 (James, 2013)
Effects of Genetically modified herbicide-tolerant (GMHT) crops on arthropods may result directly from the toxic effect of broad-spectrum herbicides or the deleterious consequences of the transgenic trait or its products, but they may be indirectly mediated via plant food resources or habitat modification (Marshall et al, 2001)
It was not possible to measure any possible effects of the transgenic trait on arthropods as the field was sown with the same GMHT variety in the three years; a direct effect of the broad-spectrum herbicide in comparison with the pre-emergence herbicide treatment was not likely to occur according to most of the records in the literature
Summary
Modified herbicide-tolerant (GMHT) maize was cultivated in millions of hectares in 2013 (James, 2013). Application of broad-spectrum herbicides may alter the composition and abundance of weed flora and other components at higher trophic levels, among which there are the arthropods (Heard et al, 2003). These provide important services in agrosystems like pollination, biological control, nutrient cycling and provision of resources for other organisms that might be interfered by drastic changes in weed management practices (Norris & Kogan, 2000). Weeds are regarded and managed as mere competitors of the crop plant, in maize, which is very susceptible to early weed competition (Bradley et al, 2000). Weeds may play other roles in agrosystems and influence higher trophic levels, such as arthropod herbivores and their natural enemies, and any alteration in the numbers, composition or phenology of weed communities may be transferred to the whole food web through several mechanisms (Smith et al, 2008)
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