Arthropods, a type of invertebrate with an exoskeleton, such as spiders, insects, and centipedes, rely on weeds for many reasons—microclimates, food, and shelter. When the weed population is tampered with it can have direct effects on arthropods that can then have indirect effects on herbivores and the food chain in general. Though genetically modified herbicide-tolerant (GMHT) crops offer new and more environmentally friendly options for weed management, they could have potential consequences on biodiversity due to a decrease of weed biomass and arthropods. Bigler et al. (2011) describe and summarize the indirect effects of weed management, specifically through GMHTs, on plant biodiversity, referencing the utilization of conservation biocontrol, which is essentially the manipulation of weed populations in order to help other components of the ecosystem, thrive. Weeds serve as the primary food source for arthropods along with providing shelter and refuge. They can also alter habitat conditions and interact with crop plants and alter their morphology, phenology, and physiology—both of which can either be beneficial or detrimental to the arthropod. In a few 3-year field studies run by the U.K. on GMHT and weed biomass, they showed that the GMHTs produced less weed biomass and seed return to the soil compared with conventional crops. Invertebrates which relied on these weeds were thus affected. Other studies showed that the impact of herbicide treatments depended on the density and diversity of weeds present and the timing and efficiency of their removal. In the end the authors concluded that the principles and mechanisms resulting in effects on non-target anthropoids and biological control functions are the same for GM and non-GM crops.¾Rachel Warburton
Bigler, F., Albajes, R., 2011, Indirect effects of genetically modified herbicide tolerant crops on biodiversity and ecosystems services: the biological control example, Journal for Consumer Protection and Food Safety, S79-S84
Albajes, R., Lumbierres B., Pons X 2009, Responsiveness of arthropod herbivores and their natural enemies to modified weed management in corn. Environ Entomol 38
Givens WA, Shaw DR, Kruger GR et al. 2009 Survey of tillage trends following the adoption of glyphosate-resistant crops. Weed Technol 23
GMHT crops benefit growers by allowing flexibility when using herbicides and making weed management simples and cheaper compared to conventional crops. Some crops such as sugar beets and maize are especially sensitive to weed competition. Due to the fact that soil conditions in the spring are not adequate for the application of herbicides the window for application is small and thus the conventional methods dictate early weed control which results in very few weeds in the entire season. Though beneficial to the farmers, the loss of weed crops later in the season when the crop is less sensitive to competition, results in a loss of arthropods and seed-eating birds.
As stated before, 3-year field studies were conducted in the U.K from 2000 to 2002 and they concluded that the management of weeds in GMHT beet and oilseed rape resulted in fewer weeds later in the season, which in turn produced less biomass and seed return compared to conventional crops. Results in a later study showed that weed biomass was generally more reduced by the early overall sprays of glyphosate, the most widely used herbicide chemical component, than by conventional herbicide programs or by later applications. The results also noted that the change in the beetles tested was due to the removal of weeds and not to the herbicides used.
Another study, done by Albajes et al. 2009 compared the abundance of arthropods in maize plots that had been twice treated with glyphosate with that in plots treated with a conventional pre-emergence treatment over a 3-year period. The authors compared three different sets of data. Firstly the number of herbivores and predators estimated by visual inspection of the plants, secondly the number of caught soil-dwelling predators and decomposers and thirdly the number of insect parasitoids caught by yellow sticky traps set out. The only plots with significant differences in weed populations showed differences in herbivore and predator plant-dwelling populations. The amount of soil-dwelling predators was consistently higher in plots covered with more weeds while the number of insect parasitoids was essentially the same between the two different situations. These results allowed the authors to conclude that only after substantial changes in weed abundance does it effect biological control on maize plants applied by plant dwelling predators and parasitoids, but that soil-dwelling predators may be more affected by even the smaller changes.
Tillage is another thing that may impact arthropod populations. Conservation tillage uses practices that minimize the disruption of the soil structure, composition and biodiversity makeup of the soil, thus minimizing soil erosion and degradation. This method is different from conventional tilling methods because its main purpose is not to create a seedbed. Tillage is an important method for weed management and different tillage techniques can alter weed biomass in different ways. GMHT technology has reduced tillage systemic use. Givens et al 2009 states that farmers in all cropping systems in the U.S increased their use of conservation tillage after adoption of herbicide resistant crops. A review of 45 studies of tillage and invertebrates showed that of the 51 arthropod species tested, 14 increased with the decrease of tilling, 15 species showed no change and 22 other species decreased with decreased tilling. Overall these results suggest that reduced tilling methods are less detrimental to the soil in which arthropods live, especially when compared to conventional tilling methods. In another study Bigler et al. (1995) tested four cropping systems (conventional, green cover, folder rye, meadow) and in the end the results demonstrated that weed management and tillage systems do in fact have a greater impact on the abundance of the predator fauna in maize and that prey consumption can be enhanced if conservation tillage is applied in conjunction with cover crops and overall sprays of herbicides.
As stated before, Bigler et al. 2011 concluded that the principles and mechanisms resulting inadvertently to arthropods and biological control functions are the same for GM and non-GM crops. The authors state that GMHT crops do offer beneficial weed management techniques along with more flexibility for herbicide use which could be a powerful tool to manipulate weeds in ways that could result in higher weed biomass. The authors also suggest that GMHT crops may increase adoption of reduced or no-tillage systems with possible positive improvements on weed management. The last suggestion is to begin with the application of reduced-tillage systems combined with later season overall sprays with adaptations to the local needs of the land. This in turn may help to enhance conversation biological control.