Pete Raal DOC Dunedin undertook the review below in answer to some questions I had about control methods for grasshopper habitat in Central Otago.
Effects of Roundup (Glyphosate), Answer (Metsulfuron-methyl), Trounce (mixture of Glyphosate and Metsulfuron-methyl) and Versatill on invertebrates
Roundup (Glyphosate)
Numerous soil bacteria, fungi, invertebrates, and other microorganisms have been studied for effects of Glyphosate application.
• There is nothing to suggest Glyphosate would adversely affect soil or terrestrial invertebrate organisms.
• Glyphosate has low acute toxicity to honey bees with a topical LD50 of greater than 100ug/bee.
• Glyphosate is readily metabolized by soil microorganisms and some species can use Glyphosate as a sole source of carbon.
• It is degraded by microbial action in both soil and water.
• Glyphosate degrades in soil, with an estimated half-life of 30 days.
• Glyphosate is highly soluble, but adsorbs rapidly and tightly to soil.
• Glyphosate has low leaching potential because it binds so tightly to soil.
Answer (Metsulfuron methyl)
Studies on the effects of metsulfuron methyl on soil biota are limited to Pseudomonas species, though there are a few studies of insects that live in soil. The lowest observed effect concentration is 5 mg/kg, based on the Pseudomonas study. At recommended use rates, no effects are expected for insects.
• Metsulfuron-methyl has low acute toxicity to honey bees with a topical LD50 of greater than 25ug/bee.
• Effects to soil microorganisms appear to be transient.
• Metsulfuron methyl degrades in soil, with a variable half-life up to 120 days.
• Half-life is decreased by the presence of organic matter though microbial degradation of Metsulfuron methyl is slow.
• Non-microbial hydrolysis is slow at high pH but rapid at lower pH.
• Adsorption to soil particles, which affects the runoff potential of Metsulfuron methyl, increased with increased pH and organic matter.
• Metsulfuron methyl has low adsorption to clay.
Trounce (mixture of Glyphosate and Metsulfuron-methyl)
• No effects are expected for insects (see Glyphosate and Metsulfuron-methyl above).
Versatill (Clopyralid)
Studies of clopyralid effects on soil invertebrates have been conducted, including field studies on the effects to microorganisms.
• Soil concentrations from USDA Forest Service applications are expected to be 1,000 less than concentrations that would cause toxic effects. Therefore, no effects to soil invertebrates or microorganisms are expected from use of clopyralid.
• Clopyralid is degraded by soil microbes, with an estimated half-life of 14 to 29 days, meaning that one-half of the amount applied remains in the soils after 90 days, one-fourth of the applied amount remains after 28 to 58 days, one –eight after 42 to 87 days, and so on.
• Increased soil moisture decreases degradation time.
• Clopyralid is weakly adsorbed and has moderate leaching potential.
Note:
LD50/LC50: Acute toxicity is commonly measured by the lethal dose (LD) or lethal concentration (LC) that causes death in 50 percent of treated laboratory animals. LD50 indicates the dose of a chemical per unit body weight of an animal and is expressed as milligrams per kilogram (mg/kg). LC50 is the concentration of a chemical per volume of air or water and is expressed as milligrams per liter (mg/L). Chemicals are highly toxic when the LD50 or LC50 value is small and practically nontoxic when the value is large.
However, the LD50 and LC50 do not reflect potential health effects such as cancer, birth defects, or reproductive toxicity that may occur at levels of exposure below those that cause death.
