Imidocloprid and Pollinators - Short and Long-term Potential Impacts
Below is an article that seems to be making its way through the state agriculture agencies... and likely elsewhere. I found several of the statements in this article regarding this compound's retention in the soil, increased effectiveness over time, and the multiplying impacts of breakdown products to be interesting and a bit disturbing in the implications this has for pollinators if used widely to treat the many species of blooming woody plants.
This article was culled from our state's landscape and integrated pest management report which goes out to many growers and landscaping firms. I read it regularly and for many pests the state is now recommending this compound and despite this article's warnings about not using it on nectar producing trees, to date there has been no warnings of such limitations in their recommendations and I can't help but think that this must be used very widely as it has so many positive attributes to the grower/nurseryman/landscaper.
Part of the reason for this article appears to be the use of this pesticide in treating Emerald Ash Borers, and, as the article states, rates have to be higher and more often to be effective. Peter Borst recently summarized why this is unlikely to be true.
I sent Peter's summary around to the USDA ARS group and got no replies, an indication of perhaps how little is known or...that there is some reluctance in discussing the matter. Its not clear to me, sitting on the sidelines, what the real situation is, but thought I would post this to the group to at least give the topic a chance for more consideration.
Again, while not a toxicologist, it would appear to me that studying the impacts of this chemical on pollinators (wild and Apis) would be reasonably straightforward....and that a partnership between biologists who study pollination and a toxicology team would perhaps make sense.
As pesticides can be a contentious issue, I would suggest making extra efforts to keep the conversation close to what facts are known and would could be done by biologists and bee people. Thanks
Your Public Servant, Sam
The article follows.....
The Facts About Systemic Insecticides And Their Impact On The Environment And Bee
By: Richard S. Cowles, Ph.D., Connecticut Agric. Exp., Valley Lab, Windsor, CT, Richard.Cowles@...
Is imidacloprid safe to use for controlling insect pests feeding on urban trees? Are insecticides like imidacloprid
responsible for Colony Collapse Disorder of honey bees? This article will try to provide some guidance and
respond to these questions.
Neonicotinoid insecticides and arboriculture
Imidacloprid is one of a growing class of insecticides (neonicotinoids) that have, since the announcement of
their discovery in 1989, become mainstays in agricultural, pest control, and landscape pest management. Two
active ingredients of this class are commonly used in arboriculture: imidacloprid (CoreTect, Merit, or Xytect)
and dinotefuran (Safari and Transtect). One of the reasons this class of insecticides has become so important
is its selective mode of action: neonicotinoids target the same acetylcholine receptor on the insect nerve
cell as nicotine (the active ingredient of tobacco), but unlike nicotine, do not bind well to the nerve cells of
humans. Therefore, it is toxic to insects and relatively nontoxic to humans and animals, including birds. Other
favorable environmental characteristics are that neonicotinoids are readily excreted by vertebrates, that they
break down quickly upon exposure to sunlight, and that they bind tightly to organic matter in soil. Another, and
probably their most important practical feature, is that they are systemic (move throughout the plant). Systemic
neonicotinoids can be applied to trees using three different application methods; these include soil applications,
systemic basal bark sprays and trunk injections. Each of these methods have their pros and cons, however, soil
and basal bark sprays are commonly used because of they are non-invasive to the tree, quick, and operational
When applied to the soil around the root system of a plant, the insecticide is absorbed by the roots and
transported in sap, where the insecticide can then reach every part of the plant. This is useful both for targeting
sap feeders (both xylem feeders like sharpshooters, and phloem feeders like aphids) and insects that feed in the
interior trunk and leaf tissues of trees, such as newly hatched emerald ash borer larvae or various leaf miners.
In contrast to broad spectrum foliar spray insecticides, systemic applications of neonicotinoids, either as soil
applications or basal bark sprays, are contained within the plant. This allows targeted control of the pest insects
rather than killing all insects, which could including beneficial predators or non-target insect species. Trials
with the neonicotinoid dinotefuran have shown that a systemic basal bark spray will provide control of armored
scale pests on Christmas trees while not impacting beneficial scale-consuming predatory beetles and parasitic
Systemic insecticides have proven their usefulness in arboriculture. Trees that would otherwise be impossible
to spray because of their great height, extremely dense foliage, or location near sensitive ecological or
human activities can be protected with systemic insecticides. For example, hemlock woolly adelgid has been
controlled in hemlocks as tall as 140-feet on trees in the Great Smoky Mountains National Park. It would be
extremely difficult to achieve this level of control with non systemic products. Furthermore, imidacloprid
was found at nearly uniform concentrations in branch samples from all levels of the crown in these large trees.
Sadly, these trees were only treated once (in 2002), and recently died because the treatment was not continued.
Research has shown that the effective dosage rates for imidacloprid are exponentially related to the diameter
of the tree trunk. As trees increase in size they require higher insecticide dosage rates to fully protect the
tree. This has been demonstrated in research trials using soil applied imidacloprid on hemlocks for control of
hemlock wooly adelgid and (Cowles, 2009) and on ash trees for control of emerald ash borer (Herms et al.,
2009). Exploring the relationship between minimum effective dosage and the size of trees for various insect
pests should be a fertile subject for further study. A deep understanding of the dose/tree size/pest relationships
can lead to optimized use of these insecticides in the environment and therefore reduce the risk of non-target
Some target pests (aphids, true bugs, and adelgids) are extremely sensitive and require very low dosages. Soil
applications of imidacloprid result in more than one year of control, and low dosages are effective. Since the
peak concentration following a soil application can be as long as 18 months later,2 it is unlikely that a tree
would need to be retreated to manage these pests for at least 2 years. Because imidacloprid and its olefin
metabolite continue to be mobilized to new growth in successive years, you may observe the population
continuing to decrease over time, to the point where the population is locally exterminated. I treated tulip
poplars at my workplace in 1995 with imidacloprid, and they have not required subsequent treatment. The rule
of thumb for these sensitive pests is to not retreat until the pest population is observed to be increasing again.
Unfortunately, borers require a much higher dosage in tissues to be effective, and any borers living in a tree
jeopardizes the long-term health of the tree. Therefore, protection from tough-to-control borers warrants annual
insecticide applications and higher treatment dosages.
Non-target effects and Colony Collapse Disorder
Probably the first non-target impact observed with imidacloprid was spider mite outbreaks in treated crops (a
phenomenon repeatedly observed in trees). Three hypotheses may explain this phenomenon; each explanation
has some supporting data. The insecticide is not poisonous to the mite, but causes secondary poisoning of
predators that feed on the mites, the insecticide acts as a “fertility drug” to the mites, and the plant is so much
healthier, that the mites can develop much better. From my own research on eastern hemlocks, I have observed
a transient outbreak in spruce spider mites that affect foliage for one year, which is more than compensated
by the improved growth of the trees when no longer weakened by adelgids. These effects may be more
pronounced when excessive dosages of imidacloprid are used relative to the size of the tree. Ecological studies
of forest hemlocks treated with imidacloprid demonstrate that it can affect many components of the insect fauna
associated with these trees.3 Such an outcome should not be surprising – after all, these systemic insecticides
are used precisely because they are potent insecticides. Hemipteran predators (such as minute pirate bugs)
are certainly eliminated with the use of systemic neonicotinoid insecticides. These and other predatory bugs
commonly feed on the sap of their target prey’s host plant, and so are subjected to direct poisoning.
The other insects for which there is great concern regarding the potential for poisoning are pollinators. While
any insect feeding on pollen or nectar could be exposed to the systemic insecticide, Colony Collapse Disorder
(CCD) has focused concern on risk to honey bees. Although the symptoms of bee poisoning with this class of
insecticides eerily resembles CCD (foraging bees become disoriented and do not return to the colony), a review
of the incidence of CCD around the world points to three or four other factors being more likely explanations.
(1) CCD has not diminished in countries where neonicotinoid insecticide use was curtailed, CCD is not found
in Australia, where neonicotinoid insecticides are used, but where Varroa mite (a parasite and vector of bee
viruses) are absent, 96% of colonies with CCD have been found to harbor a complex of viruses, for which
Israeli Acute Paralysis Virus is most strongly implicated; and hive equipment from CCD colonies can be
disinfected through irradiation, which implicates involvement of a pathogen. For tree species such as Fraxinus
(ash trees) which are not pollinated by bees or that are not visited by pollinators, systemic treatments will have
little to no impact on pollinator species.
The evidence pointing to other factors as likely causes for CCD does not leave neonicotinoid insecticides off the
hook for their potential to poison bees. The facts below are things that practitioners should consider:
• Neonicotinoid insecticides used in arboriculture are highly toxic to bees when exposed to a direct spray
application. For example, imidacloprid and dinotefuran have acute LD50s for bees of 18 and 75 ng per
• Exposure of insects to low neonicotinoid concentrations (well below their acute LD50) can cause mal
adaptive and ultimately lethal behaviors.
• Imidacloprid is readily metabolized in trees to imidacloprid olefin,2 which is 10 – 16 times more toxic to
insects than the parent compound.
• Peak concentrations of imidacloprid are not reached in some trees until about 18 months after a soil
application, which means that trees treated every year could accumulate concentrations toxic to bees
over several years.
• Arboricultural use concentrates these insecticides compared with agricultural uses. For example, the
maximum dosage for treating two 32-inch dbh trees with some imidacloprid products is equivalent to
treating one acre of agricultural crops.
• Higher concentration in plant tissues may increase risk to pollinators.
Little is known about the actual concentrations of these insecticides in nectar or pollen from treated landscape
trees. At this point, arborists should mitigate these concerns by adjusting how they treat trees, how often trees
are treated, and by choosing the most appropriate product. Risk of bee poisoning integrates components of
intrinsic toxicity (just how much of the insecticide is required to cause adverse effects in bees), and their degree
of exposure to that poison.
Arborists can avoid exposing pollinators by avoiding treating tree species that are highly attractive to pollinators
(linden, tulip poplar, Korean Evodia and catalpa, for example) with systemic insecticides. If trees attractive to
pollinators do require treating with a systemic insecticide, dinotefuran applied immediately after bloom may be
safer to use than imidacloprid products. Whereas imidacloprid can be detected in hemlock foliage for about 8
years after soil injection, preliminary data from various tree species suggest that dinotefuran breaks down over
the course of one growing season. Therefore, if the pest actively feeds following bloom of a tree species, then
a dinotefuran application after bloom can quickly target that pest, and then residues should dissipate so that it is
not present in pollen or nectar at biologically relevant concentrations the next time that plant blooms.
Risk of soil applied neonicotinoids leaching into groundwater
Another concern with soil applied systemic insecticides is that they may pose a risk of leaching to groundwater
or to nearby ponds and streams. This is really a non-issue when using these products in most urban landscape
soils. Both imidacloprid and dinotefuran do bind to organic matter in the soil and most urban landscape soils
with mature trees have higher than 3% organic matter. Therefore, there will be little risk of leaching as long as
there is a fair degree of organic matter in the soil (2% or greater), the insecticide is not placed below the organic
horizon of soil (as might happen with a deep root feeder probe), and the insecticide is not applied in such
concentrated “spots” that the active ingredient will exceed the binding capacity of the soil. Therefore, I suggest
that practitioners use very shallow subsurface (2-4 inches) application of systemic insecticides, dispersed near
the trunk of the tree. For high dose applications, expanding the area of soil treated near the base of the trunk of
the tree may be important to guarantee that the binding capacity of the organic matter is not exceeded. A novel
application technique to consider for high volume treatments is to use a hose-end sprayer to disperse the active
ingredient around the base of the tree, which should then be incorporated with an additional light watering to
wash the residues from the soil surface. In all of my experiments, I was unable to cause imidacloprid to leach
more than a few inches through an organic soil layer found under forest hemlocks, even with a one inch per
day irrigation protocol adding water to soil columns. Dinotefuran has much lower organic matter binding than
imidacloprid, and so it does pose a greater risk for leaching (though this risk may not be great). However,
dinotefuran can be successfully applied as a basal bark spray. It is surprising how quickly this active ingredient
is absorbed through the bark and is then transported to the foliage. My trials in eastern hemlocks have
demonstrated this approach to be equivalent to soil injection of the same quantity of product, and in conditions
where the soil is dry, compacted, or excessively wet, a trunk spray could be more effective than soil injection.
While neonicotinoids should not be applied to trees growing directly in water or to areas where surface water is
present there is little risk of these products leaching into groundwater when applied correctly to the majority of
soil types across the United States.
Imidacloprid and dinotefuran are very effective tools for managing many insect pests of landscape and forest
trees. Choosing the right product for the job and applying the product carefully can protect both the trees that
your customers value and the environment.
Sam and others- this seems to me to be a well-reasoned, objective article for a systemic insecticide whose convincing yet contradictory evidence for impacts on bees multiplies like rabbits. For arborists, I agree, avoid its use wherever possible on flowering trees used by bees (when speaking to the pesticide certification workshops here in northern Utah, for instance, I advise against use on redbuds, willows and basswoods). I keep trying to come to a resolution in my own mind about use of these neonicotinoids in agriculture. Yet at posters and talks at meetings, and conversations with presenters, I keep finding conflicting yet persuasive evidence from capable scientists, most recently at the International Pollination Workshop at Penn State University and at the national Entomology meetings in San Diego. One can hope that explanations for the contradictions are more than idiosyncratic combinations of application method, crop, climate and soil. If any capable toxicologists see patterns, we should all be interested, especially for cases where these compounds would be displacing insecticides known to be lethal to bees. When are they safer than current practices, when aren’t they?
James H. Cane