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.

Plague, prairie dogs and pesticide policy

Urban pest control encompasses an incredibly wide range of organisms including insects, arachnids, birds, bats and a smorgasbord of urban wildlife. I was reminded of this yesterday with a call from a PMP seeking help with flea control in prairie dog towns. It seems there is a plague epidemic among the prairie dogs living in and around Santa Fe, New Mexico.

Since plague is not a common phenomenon in the United States, a little background might be helpful. Plague is a disease caused by a bacterium called Yersinia pestis. The bacterium can be transmitted through the bite of an infected flea, through handling or skinning an infected animal or through the air from an infected human (or more frequently, cat) similar to flu. The term "plague" stirs most people because of its association with the black death of Europe, when dozens of millions died over a 300 year period starting in the 14th century.

There are two ecological forms of plague: urban plague, spread largely by commensal rats; and sylvatic plague, which is spread largely by wild rodents in rural areas. Both forms of plague remain a threat in this country, though the last outbreak of urban plague occurred in Los Angeles in the 1920s. Most recent U.S. cases of plague, 10-15 a year, are sylvatically transmitted and occur over scattered sites in the western states, including a few in Texas.

Rodent control by PMPs has, without a doubt, helped keep urban forms of plague rare over the past century. But effective pest control for public health is still needed to control of sylvatic plague in some areas. In the current Santa Fe epidemic, for example, one of the affected prairie dog colonies is adjacent to a school playground in an urban neighborhood.

Unfortunately, some citizens and politicians appear not to appreciate the seriousness of arthropod-borne disease. Perhaps this is because of our success in keeping arthropod-borne disease rare. Maybe it's a need for better public health education or a lack of understanding of science and medicine. Whatever the reason, in recent years I see more instances where vague health and environmental concerns seem to be trumping well-documented and real concerns about insect-vectored disease.

In Santa Fe, citizen pressure in recent years has led to schools and the city replacing effective conventional pesticides with generally untested, organic alternatives. One specific consequence of this approach is that city workers are being encouraged to use diatomaceous earth for control of Xenopsylla cheopsis, oriental rat flea, in prairie dog burrows. In a cursory review of the literature I could find no information to justify use of diatomaceous earth for flea control in prairie dog burrows. I was, however, able to quickly find peer-reviewed research on the effectiveness of field applications to prairie dog burrows of several other insecticides including carbaryl, DDVP, permethrin, deltamethrin, and pyriproxifen.

While it can be argued that none of these products provide an ideal, long-lasting or inexpensive solution to plague-bearing fleas, they do provide significant, quick control of fleas. They can also rapidly prevent or eliminate plague outbreaks in prairie dog towns. And at least some of these tested products should be acceptable to the staunchest pesticide critics. Pyriproxifen, in particular, is a low toxicity insect growth regulator that has shown excellent flea control in prairie dog burrows with minimal ecological disruption. Highly specific oral vaccines for prairie dogs are also being tested with promise for providing a longer-term solution to plague outbreaks.

On the other hand, my own work several years ago with cat fleas showed that although diatomaceous earth can control fleas under dry laboratory conditions, the addition of moisture to treated soil quickly renders treatments ineffective for control of adult and larval fleas. This casts serious doubt on the effectiveness of field applications of diatomaceous earth dust into damp underground burrows. At the very least it points out the need for field testing before recommending such treatments.

So why would a respectable community persist in requiring an untested, most-likely ineffective pesticide to control plague-bearing fleas? In this case it seems to be because anti-pesticide advocates, armed with little more than an easily-transmitted fear of technology, are able to persuade policy makers that "natural" is always better and safer. The same strategy is being used in my community to spread fear about the responsible, professional use of mosquito spray campaigns in areas of high West Nile virus risk. The big problem here is that untested recommendations are being taken seriously by government and public health.

It is one thing to choose to use untested home recipes in a backyard garden, where the worst that can happen is for the gardener to lose a few of his or her own plants. It's another thing to pressure communities to stop using proven pest control technology against pests that threaten community health. It make sense that before a community switches from a proven technology, advocates should provide (1) proof that the current technology poses an environmental or health cost that exceeds the value of the benefits provided, and (2) evidence that alternatives are both safer and equally effective as the current technology.

Decision-makers at the school, local and state level need to understand both the science and the stakes involved before restricting the kinds of pesticides that can be used by pest management professionals. This is what I believe is wrong with the growing crop of local ordinances banning municipal governments from using certain classes of pesticides in parks and schools and other public places.

You will get little argument from me that pesticides pose their own share of unique environmental and health challenges, but the problem is often less than the public is led to believe. And in many cases, the negative impacts of pesticides can be blunted by keeping them in the hands of professionals and using them in the context of a well thought-out IPM program. Our ability to control pests quickly at reasonable cost, often with a high degree of selectivity and environmental compatibility, means that pesticides will continue to be a valuable part of pest control in the foreseeable future.

I do not live in Santa Fe, and cannot speak to all the issues and circumstances of their plague management program there. But if I were a parent with a child in school next to a prairie dog town with a known plague outbreak, I know I would want a control option that had been tested and shown effective.

Playing without Pesticides?

In case you've not heard, there's a movement afoot to ban pesticides as part of a green approach to community improvement. Pesticide bans are most common in schools and parks, but are even being proposed (most famously in Toronto, Ontario) for whole cities. A recent news story from New Jersey tells of a school in Mansfield, NJ that has taken to posting "Pesticide Free" signs near the playgrounds to advertise the school's policy of not using pesticides (at least around playgrounds).


Such actions, in my opinion, are misguided and teach kids the wrong lessons. Those of you who love kids and the environment, like I do, please hear me out before you click the "close" button on your browsers.


Let's ask ourselves, what message do pesticide bans really convey? Let me propose a few.

• All pesticides are bad for the environment. This message ignores the diversity of pesticide products, all of which are different in their modes of action and degrees of toxicity. A pesticide is, by definition, something that manages pests, not something that is dangerous for the environment. The environmental impacts of today's pesticides are significantly different than the products that Rachel Carson warned us about in the early 1960s. Few of today's pesticides, for one example, have the potential to accumulate in the food chain.


• All pesticides are unhealthy for kids and the rest of us. Not true. The older, nerve-toxin targeted pesticides have undoubtedly contributed to this misconception; but there are lots of pesticides that pose very low risks to humans and non-targeted organisms.Over the past twenty years or so, especially, pesticide toxicity and persistence has decreased dramatically.


• You can't trust science-based regulatory agencies to keep unsafe pesticides out of our communities. While I'm not ready to go to bat for all our federal regulatory agencies, few people fully appreciate the vetting process that U.S. pesticides go through before making it to market. Suffice it to say here that pesticides go through much more thorough and expensive safety testing than most other consumer-destined products.


• We don't really need pesticides. This message denies the value that pesticides have historically provided in keeping food costs down (for the poor as well as the rest of our society), saving human lives by keeping disease-carrying pests at bay, improving safety and appearance of athletic fields (ask your kid's high school coach about this one), and maintaining healthy and comfortable environments for all of us (care to share your sleeping space with bed bugs anyone?).

Leonard Douglen, Executive Director of the New Jersey Pest Management Association, made a good point in a recent editorial when he observed, "When one considers the many insect pests that can attack children, from ticks that can cause Lyme disease to mosquitoes that transmit West Nile Fever, cockroaches that can spread a variety of diseases, as well as stinging insects, the need for professional pest control becomes self-evident."

One final observation. Pesticide bans may be more palatable to consumers, and seem more acheivable, in cooler, northern communities where outdoor pest problems are less common and less severe. In the southern states the warm season is longer, weeds grow faster, and insect and plant disease pressure is significantly greater than, say, Toronto. Under these conditions, pest management is more essential to maintaining safe and attractive landscapes. Try taking the average Texan's bag of fire ant bait away from him or her, and you'll quickly understand what I mean.

Legislators should be careful lest they think that pesticide bans are the way to go. I propose a more thoughtful and nuanced approach. It may not be as fashionable or simple a message as "Pesticide Free", but requiring use of integrated pest management, and making sure that pesticide applicators are well trained in the safe and judicious use of pesticides will, in the long run, be best for us and our environment.