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Additional benefits of neonics

Thiamethoxam, a broad-spectrum neonicotinoid insecticide contributes to better seedling vigour compared (as compared to no treatment) says Clarence Swanton at the University of Guelph.

Do certain seed treatments go beyond protecting young plants from insect pests? That’s an important question, especially if the seed treatment is a neonicotinoid insecticide. Neonics are currently under intense scrutiny by government agencies in many countries and their use is being restricted in some jurisdictions.

It’s clear that at least one neonic seed treatment seems to provide more than insect pest protection in corn, but just what protection it provides and how it does so hasn’t been completely clear.

“Thiamethoxam is a broad-spectrum neonicotinoid insecticide that, in seed treatment form, contributes to better seedling vigour compared to no treatment,” Clarence Swanton, a professor in the department of plant agriculture at the University of Guelph (U of G), says. Thiamethoxam controls a wide variety of commercially important crop pests, and is used as a foliar spray or soil treatment (Actara), or as a seed treatment (contained within Cruiser).

“When thiamethoxam is applied to seed, we see increased germination rates, faster root growth, greater seedling heights and more biomass accumulation, but the physiological mechanisms by which these enhancements occur is not well known,” Swanton explains. “Other researchers have measured the ability of thiamethoxam to do things such as increase the antioxidant capacity of a certain molecule found in corn seedlings, called salicylic acid, which is an antioxidant that plays an important role in the defence against plant pathogens. It is also able to improve plant response to abiotic and biotic stresses, including those caused by the presence of weeds.”

However, thiamethoxam seed treatment may be helping seedlings perform better because it reduces the amount of hydrogen peroxide (H202), a free radical that can accumulate in a seedling due to the stress of having weeds nearby. (Free radicals cause damage in plant and animal cells through a process called oxidation.)

“Thiamethoxam may be elevating the expression of genes involved in natural scavenging and destroying of H202, in addition to genes involved in other metabolic pathways. This is what we wanted to find out more about,” Swanton says.

Swanton and his colleagues have investigated how much better corn seedlings perform with weed pressure, with and without thiamethoxam as a seed treatment. They conducted measurement and analysis at the plant (macro) level, as well as at the molecular level.

In addition to Swanton, the team included Maha Afifi, Elizabeth Lee and Lewis Lukens, a research team at the department of plant agriculture at the U of G. In a laboratory environment, thiamethoxam-treated seeds were planted, with some of the resulting seedlings growing up in the presence of neighbouring weeds (a perennial ryegrass). The researchers harvested seedlings at the fourth-leaf-tip stage, washed the roots, and counted and measured crown roots. Shoots and the entire root system were then bagged separately and dried to determine total shoot and root biomass. Other seedlings were harvested for physiological and molecular analysis.

“At the macro level, we found the treated corn seedlings showed enhanced root development and seedling vigour, with none of the shade avoidance characteristics that typically develop when there are neighbouring weeds present,” Swanton explains. “We believe this was a result of morphological, physiological and molecular processes. This is the first report to identify the mode of action of thiamethoxam within the physiological mechanisms of early crop and weed competition.

“In short, our results suggest thiamethoxam enables corn seedlings to maintain their antioxidant protective system to avoid damage caused by oxidative stress from neighbouring weeds,” Swanton says.

Swanton, Afifi, Lee and Lukens found thiamethoxam reduced H202 accumulation, as well as the subsequent damage caused to cells by its accumulation. “It seems to accomplish this through boosting the capacity of genes involved in scavenging this free radical,” Swanton says. “Preventing the accumulation of H202 and enhancing the entire antioxidant system means the plant experiences less cellular damage caused by abiotic and biotic stresses, such as lower light levels caused by neighbouring weeds. The plants from treated seed don’t have to expend as much energy for cellular repair and the energy can therefore be used for growth and maintenance of plant tissues. So, these results suggest plants from thiamethoxam-treated seeds may be better adapted for survival under harsh environmental conditions.”

Swanton believes these results have several other implications for the role of seed treatments in agriculture.

“Normally, seed treatments are thought of only in terms of insect and disease control, but the results of this study suggest it may be very worthwhile to explore entirely new chemistries and new modes of action in novel seed treatments to enhance free radical scavenging and activate genes involved in the antioxidant defence system,” he says. “It’s clear from our study and the work of other researchers that some seed treatments have this capacity, and that may be critical in the development of crop hybrids and cultivars that are more stress tolerant to weed competition.”

The researchers are now investigating whether soybean seedlings grown from thiamethoxam-treated seed will demonstrate the same responses to weed pressure as those of corn seedlings.