Report Retort

On January 14, Vermont’s Endangered Species Committee held a five hour virtual meeting. Three and a half of those hours were devoted to a discussion of the risk to endangered and threatened bats from roadside spraying of chemical pesticides in the BLSG Insect Control District. More than 25 people participated in the meeting.

An imagined grid of the Teams meeting.

Almost all the biologists who have addressed the issue of injury to endangered bats in the BLSG District seem to agree that a real risk exists when a mist of concentrated droplets of malathion or permethrin hovers on summer nights above roads through Vermont’s most important bat feeding-habitat. Alyssa Bennett, Vermont’s small mammal biologist, agreed with this consensus at the meeting:

“I would consider this as a threat, we would identify this as a threat in our Wildlife Action Plan, and I would think of this as something that could have cumulative impacts.”

But Bennett did not linger on that consensus. She works for the Vermont Department of Fish & Wildlife and had apparently been given an assignment to discredit a report that made the scientific case the independent biologists agree with. The 20-page report from Arrowwood Environmental concludes that bats are uniquely vulnerable to BLSG’s pesticide operation and that previous assessments probably underestimate that vulnerability.

Bennett’s critique of the Arrowwood Report was calm, authoritative, and deeply flawed. Most of Bennett’s comments either misinterpreted the literature, mischaracterized the report, or were otherwise inapt or off-target.

Misinterpreting the scientific literature

An important argument made in the Arrowwood Report is that most of our understanding of the pesticide risk to bats is based on studies of small mammals other than bats and sometimes even birds. For several reasons, pesticide researchers rarely use bats as their test subjects and rarely do field studies (as opposed to laboratory studies) with bats. This limits our ability to know the exact risk of pesticide exposure to wild bats. The Arrowwood Report makes a convincing argument that unique features of bat anatomy, physiology, and ecology make bats substantially more susceptible to pesticides than are the common surrogate animals, and that pesticide exposure in the field can have more serious impacts than lab studies suggest. The strong message is that bats are probably harmed by pesticides more easily than the limited surrogate or laboratory studies suggest.

Bennett seems to have missed this point and repeatedly berates the Arrowwood Report for relying on limited data derived from laboratory or surrogate research:

“I wasn’t able to find in either the EPA or the references in the [Arrowwood Report] any direct research or survey work, or data collection that indicates the effects of malathion on bats in a laboratory or field setting.”

“…malathion could potentially effect bat behavior by interrupting locomotion, grip strength, and feeding behavior and in this case the biological opinion stated that they only had medium confidence in this effect based on the fact that all of the research with malathion had actually been done in other orders of mammals so not even any type of bat at all.”

It is exactly this limitation of the data that requires us to be more concerned about exposing bats to pesticides. The fact that we do not know how dangerous malathion is to bats does not mean that it is not dangerous. In fact, we know malathion is highly toxic to many types of animals (including humans) and the only reason we are unsure about bats is that the studies have not yet been done.

The Arrowwood Report cited an important article about this issue (Hernández-Jerez et al. 2019). The article concluded “that bats are not adequately covered by the current risk assessment approach, and that there is a need to develop a bat‐specific risk assessment scheme.” This article argued carefully that the unique biology of bats makes them more vulnerable to pesticides than previous studies suggest. For example, it points out that the wing membrane of bats is highly susceptible to absorbing substances like pesticides, and that bat grooming (licking each other’s fur) is known to distribute contaminants through bat colonies. Bennett seems to ignore these points and mischaracterizes the article’s conclusions:

“They [Hernández-Jerez et al. 2019] actually concluded that we don’t have enough information to assess the level of risk from these other methods of transmission [absorption through the wing membrane, inhalation, grooming, mothers’ milk].”

Although it would be good to have more information about bats being exposed to toxic pesticides through absorption by the wing membrane, inhalation, grooming, and mothers’ milk, biologists already know that this is a unique combination of traits and is potentially deadly for bats. This is likely especially true when bats fly through a mist of pesticide droplets lingering over their prime feeding grounds, as happens in the BLSG District.

Many of the other critiques Bennett made were different versions of this claim that evidence was lacking, without acknowledging that biologists familiar with bats are concerned about their unique vulnerabilities to pesticides. The lack of perfect evidence seems to be poor justification for refusing to protect endangered bats from BLSG’s practices.

Mischaracterizations of the Arrowwood Report

One example of a misplaced criticism of the Arrowwood Report was Bennett’s claim that it conflates the pesticides used by BLSG with organochlorines which are not used for mosquito control in Vermont:

“But a lot of references that I was able to find and that are in the [Arrowwood Report] really drawing conclusions about the potential impacts of these toxins are referring often to organochlorines.”

It is irrelevant that Bennett was able to find literature about the effect of organochlorines on bats. It is also irrelevant that some of the references cited in the Arrowwood Report refer to organochlorines. Many articles about the effects of pesticides on bats refer to organochlorines in addition to organophosphates (e.g., malathion) and pyrethroids (e.g., permethrin). The Arrowwood Report makes no arguments about organochlorines and never conflates them with malathion or permethrin.

Bennett criticized the Arrowwood Report for citing “secondary” references:

“When I have gone through and done some spot-checking of some of the references that are in the [Arrowwood Report] that did sometimes lead to what appear to be secondary sources that were referenced as opposed to actual primary research.”

Only one example of this was given and the uncited “primary” reference was an unpublished master’s thesis. Bennett claimed that it included information that was not referred to in the Arrowwood Report, but it is unclear why this claim was made unless its goal was to unfairly disparage the competency of the report’s author.

Inapt comments

Bennett reported on some literature that was not cited in the Arrowwood Report including a study in which the authors looked for pesticides in bats and found no malathion or permethrin:

“Sparks in 2006… was looking for organophosphates in bats in Indiana that were submitted for rabies testing and they did find low levels of other insecticides but they specifically said that they didn’t find malathion or any of the pyrethroid insecticides.”

It is difficult to understand why this might be relevant because we do not know anything about the exposure of these bats to any pesticides. But it is encouraging that some bats have no detectable body burden of these pesticides.

Bennett shared data from her Champlain Valley monitoring of the populations of little brown bats, a state threatened species:

“In the decade we have been doing that since the decline of white nose syndrome … we are seeing colony sizes that are stable and, in some cases, increasing and that includes a lot of the colonies that are in towns within the spray district.”

This is good news and was apparently presented to suggest that listed bats are doing just fine even where BLSG sprays pesticides. The suggestion that this might be evidence the pesticides have not inhibited the recovery of one species is undermined by Bennett’s finding that the other four state-listed bat species are still so rare in the BLSG District that she cannot conclude anything about their population status.

This type of comment seems to reveal that the primary goal of Bennett’s presentation was not to analyze the science in the Arrowwood Report but to sow doubt about the report’s credibility.

Red herrings

Bennett reported on articles about permethrin and bat houses that were not discussed in the Arrowwood Report. Experiments compared pipistrelle bats (in Great Britain) living in bat houses constructed from wood treated with pentachlorophenol or wood treated with permethrin. Only the pentachlorophenol caused injury and death to the bats. Bennett emphasized more than once that this was important information suggesting that permethrin might not be very toxic to bats. However, one article (Shore et al. 2001) reported that “Permethrin was not detectable in body washes and tissues of bats exposed to PCP/permethrin mixture or permethrin.” After a month in the treated houses, the complete absence of permethrin on the fur or skin of bats or in their tissues raised some question about how much exposure the bats had to permethrin. The authors explain that the permethrin apparently was not rubbed off the wood by the bats and also did not evaporate quickly so the bats did not inhale it. In other words, bats were not harmed by permethrin because they were not exposed to it.

Introducing this research as evidence that permethrin is not harmful to bats is highly inappropriate. It only distracts from the important question of how bats might be harmed when repeatedly flying through, and catching insects within, a mist of droplets of concentrated permethrin. 

After the January meeting Bennett shared an article titled Bats and Fog (Pye, 1971). The article hypothesizes that bats will not fly into fog because ultrasonic resonance from the tiny water droplets interferes with their echolocation. This would be important evidence that BLSG’s pesticide mist cannot harm bats because they will detect it and then avoid it. The article presents convincing evidence that natural fog might be a problem for bats, but also presents enough information to discern that BLSG’s pesticide mist will not interfere with the echolocation of Vermont’s bats.

A figure from Pye (1971) annotated (in red) with the target range of droplet diameters of the truck-mounted ultra low volume pesticide sprayers used by BLSG, and the range of typical frequencies of the little brown bat. ULV spray droplets are smaller than most natural fogs and will resonate at ultrasonic frequencies well above those detected or produced by any of Vermont’s bats.

The ultra low volume pesticide sprayers used by BLSG produce an engineered mist of droplets and target a narrow range of droplet diameters (8-15µm, Mount 1988). This range of diameters will resonate with ultrasonic pulses but according to Pye’s analysis only at much higher frequencies (192-494kHz) than any of Vermont’s bats can detect or produce (see figure above). Therefore BLSG’s pesticide mist will be completely “invisible” to our bats and cannot interfere with their echolocation.

This is another article that should not have been part of the discussion among busy scientists attempting to understand a difficult topic and make a well-informed decision.

I am puzzled by the Fish & Wildlife Department’s use of repeated misreadings of the science to justify opposition to protecting endangered animals — one of the primary missions of the Department.

Stand by and stand up for science

The Endangered Species Committee’s Scientific Advisory Group on Mammals (primarily a group of independent biologists volunteering their time) has reviewed the Arrowwood Report and in September 2020 voted 4 to 1 in support of its conclusions that there is sufficient risk to listed bats from BLSG’s pesticide spraying that something should be done about it. At the January 2021 meeting the biologists on the Endangered Species Committee proper did not take a vote on this matter. This vote could happen soon, and 18 months after the Commissioner of Fish & Wildlife first received the Arrowwood Report, it will not be soon enough.

References cited

Hernández-Jerez, A, P. Adriaanse, A. Aldrich, P. Berny, T. Coja, S. Duquesne, A. L. Gimsing, M. Marina, M. Millet, O. Pelkonen, S. Pieper, A. Tiktak, I. Tzoulaki, A. Widenfalk, G. Wolterink, D. Russo, F. Streissl, and C. Topping. 2019. Scientific statement on the coverage of bats by the current pesticide risk assessment for birds and mammals. EFSA Journal, 17(7):5758, 81.

Mount, G. A. 1998. A critical review of ultralow-volume aerosols of insecticide applied with vehicle-mounted generators for adult mosquito control. J Am Mosq Control Assoc 14:305–334.

Pye JD. 1971. Bats and fog. Nature. Feb 19;229(5286):572-4. doi: 10.1038/229572b0. PMID: 4925363.

Shore, R.F., D.G. Myhill, C. French, D.V. Leach, and R.E. Stebbings. 2001. Toxicity and tissue distribution of pentachlorophenol and permethrin in pipistrelle bats experimentally exposed to treated lumber. Environmental Pollution 73(2):101-118.

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