Critiquing insecticides as a last resort

Pesticides may not always be the worst environmental choice
when considering pest control options, according to a
recent scientific society position paper.

Two weeks ago I wrote about a recent position paper by the Entomological Society of America and its sister societies, American Phytopathological Society and the Weed Science Society of America. The paper critiqued some popular integrated pest management (IPM) concepts, including the idea that IPM programs should use only "least toxic" pesticides.  The same paper also addressed the widely adopted idea that pesticides should only be used "as a last resort" in an IPM program. Today I wanted to elaborate on use of pestcides as the last resort.

The concept of "last resort" pesticides is tricky because it's meaning is open to different interpretations. On the one hand, using pesticides as a last resort could mean that pesticides are used only when pest numbers or damage from exceed some pre-determined threshold. The use of thresholds is usually invoked in combination with a pest monitoring program and with an integrated control strategy including non-chemical tactics such as habitat modification, sanitation and quarantine. This is a proven IPM approach that has been used since the earliest days of IPM.  As the Societies' position paper points out, however, this is not the only possible interpretation.

Many agencies and regulators have interpreted the "last resort" imperative to mean that when a pest problem occurs, all non-chemical controls must first be attempted. Only when all else fails, should pesticides be considered. There is a subtle but important difference in these two interpretations. In the latter case, thresholds are made secondary to a process of trying first one non-chemical control, then another and then another, before finally turning to a pesticide. Such delaying tactics, according to the statement, can lead to unacceptable pest buildups and damage.

From my perspective, this approach was used last summer in my community during an outbreak of mosquito-borne, West Nile virus (WNV). While medical doctors in our city were worried that too many human cases of WNV had already occurred, and that aerial pesticide spraying was needed break the epidemic, those in the community with deep misgivings about pesticide use were insisting that not enough had been done in the areas of public education, breeding site removal, and larvaciding. They requested that the cities and county wait until everything that could conceivably be done before resorting to spray planes.  They were essentially invoking the "last resort" concept.

While this approach may seem reasonable on the surface, it's consideration resulted in delay and ultimately nearly 400 (serious) human cases of the virus in my county alone.  A predetermined threshold (say, to commence aerial spraying when education, source reduction and ground sprays fail to prevent mosquito infection rates from reaching levels likely to result in human cases) would have alerted the community that preventive tactics had already failed, and more aggressive controls were needed.

The ESA position paper provides links to a number of similar cases where following the "last resort" approach led to out of control pest problems and higher control costs. Many of these examples illustrate that waiting to control a pest may result in missing the ideal time to treat, and may ultimately result in the need for higher concentrations and more applications of pesticides to control a pest infestation.

Finally the position paper points out that the "last resort" concept implies that pesticides are always the worst choice, which is not true. Non-chemical techniques are sometimes more expensive, require more labor and energy, and result in lower levels of control than chemical pesticides. An example cited from Texas was conventional tillage to control weeds in arid farmland.  A study showed that herbicide use not only was more effective in conserving critical water, but it was less costly.

Rather than talking about "using only least toxic pesticides as a last resort", I believe we remain truer to the original spirit of IPM if we talk about "minimizing risks" and "using pesticides only when necessary" to prevent or control damaging levels of pests. Integrated pest management has never been easily distilled to a simple formula or prescription.  Each pest and situation is unique and deserves a flexible approach to do the right thing. This flexibility of IPM is both its greatest strength and, perhaps, its greatest weakness.  Flexibility is a weakness because IPM is difficult to codify, and develop rules for that fit all situations. But flexibility is a strength because IPM is adaptable to many settings--not just agriculture for which it was originally developed.