What is a disadvantage of a contact herbicide?

A contact herbicide is designed to kill or damage plants upon direct contact and typically works by affecting only the parts of the plant that the herbicide touches. The correct answer highlights that contact herbicides often require repeated applications for lasting effects because they do not offer residual activity once they dry or are washed away, meaning they provide only a short-term solution. This necessitates multiple treatments to manage the targeted plants effectively, especially if any new growth emerges or if part of the plant is missed during application.

The other options, while they describe potential issues, do not accurately capture a fundamental disadvantage of contact herbicides. For instance, being less effective in low concentrations speaks to the need for appropriate usage rates rather than being an inherent disadvantage of contact herbicides specifically. The specificity in affecting only certain plant types may also not be as relevant since many contact herbicides are capable of impacting a wide range of plants, but efficacy can vary among different species. Concerns about soil erosion are more associated with management practices rather than being a direct disadvantage of the herbicide itself.