Neonicotinoids are chemical products that are effective in systemic control of insect pests in plants. Systemic application of insecticide has multiple advantages over other methods, such as topical applications. When applied systemically, neonicotinoids are absorbed by the plant, and persist for a longer period of time, so that insect pests subsequently feeding on plant parts ingest the chemical and die.
Because of this mode of action, there has been concern about neonicotinoids persisting long enough to be found in plant products such as nectar or pollen, where they could be ingested by foraging bees.
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I have been following this topic for several years, collecting various articles that have been trickling in. As licensed commercial applicators, we are trained that “the label is the law”, so pesticide applications to plants are always made according to label instructions. However, there have been claims made about neonicotinoids causing problems for Honeybee populations.
I have personally attended seminars here in Ohio where scientists leading research on neonicotinoid use for controlling invasive insect pests have stated that despite some claims to the opposite, there have been “no documented studies” proving a relationship between neonicotinoid use and Honeybee decline. Since then I have read some articles referring to studies that seem to show a correlation, but no conclusive proof. As an applicator who cares about the welfare of beneficial insects, the lack of conclusive studies is a source of frustration. In fact, the approach I was leaning toward was “if there is a high level of uncertainty, perhaps it is best to seek alternatives for treatment, or to not treat at all”. There is a lot to consider when deciding whether or not to treat a plant chemically, but that topic is beyond the scope of this article.
What is the issue with neonicotinoid use and bees? The reason neonicotinoids are great options for pest control are ease of application, larger windows of application timing, and more targeted applications. Using topical sprays to control pests requires very specific timing (pest usually needs to be present at the time of spray), and there is a risk that other non-target plants may get sprayed (wind drift). Also, non-targeted insects present at the time of application can come into contact with the chemical.
A systemic application is much simpler: no spraying may be required (usually applied as liquid drenches into the soil around the root zone of the target plant, or injected directly into the tree), only insects feeding on plant tissue are targeted, and timing is much less critical since the chemical resides within the tree, eventually breaking down so reapplication is necessary for continued control. And this is the question raised by many concerned people: If the plant tissue has enough chemical to kill insects feeding on the plant, isn’t it possible that the nectar produced by the plant’s flowers will be laced with the chemical as well? So how does this affect bees? These are very reasonable questions, and I agree that they merit consideration.
I was intrigued to see Bernadette Mach (Department of Entomology, University of Kentucky) on the speaker schedule at the Ohio Chapter ISA Tree Care Conference in Columbus, Ohio, this year. Intrigued, because she was speaking on the topic of neonicotinoid levels that persist within a plant after treatment, specifically in nectar. Her talk was in two parts, both of which applied to our concerns as commercial applicators. The first half dealt with her studies on which flowering plants were attractive to bees. The second half was measuring residual levels of systemic chemicals in nectar following application at label rates during different times of the year. Details of her study informing the second half of her talk can be found online (Uptake and Dissipation of Neonicotinoid Residues in Nectar and Foliage of Systemically Treated Woody Landscape Plants).
In short, there were several important details that I came away with:
- Of two chemicals commonly used, one has a lower residual in nectar than the other.
- The timing of the application matters. Applications made post-bloom had a low residual in nectar the following year.
- For some plants, the issue does not matter. For example, Boxwood, in particular, has caused me difficulty. Leafminer is a very common insect pest that can destroy Boxwoods, but it can be readily treated with a systemic application. Last season I began communicating with clients my concern over treating Boxwoods and potential harm to bees. After communicating with Bernadette specifically on this topic, I feel more comfortable continuing with Boxwood treatments as usual. The reason is that Boxwoods are not a plant that attracts bees, so there is a greatly reduced risk that they would feed on the nectar.
As a Certified Arborist and licensed commercial applicator making recommendations to clients, I need to consider each plant prior to treatment. Yes, the client wants to control Magnolia Scale, but should we attempt to do so systemically? Is this plant attractive to bees? If so, should we time the application post bloom? Will this concur with the insect life stage being targeted? What about other options? Can we apply a targeted topical spray that will be effective?
In some cases, nothing more will be possible than to explain to the client the implications of treatment. For example, if the client has a Hawthorn that consistently looks ugly each season when the leaves turn brown and blotchy due to Hawthorn leafminer, I can explain that the best way to control this pest is with a systemic application in the fall (which is essentially pre-bloom) to protect the tree from the pest in spring. This means that there will be higher levels of neonicotinoid in the nectar the following spring, and since bees are attracted to Hawthorn blooms there is a chance they may be affected by the chemical. Leafminer in Hawthorn causes damage that is mostly aesthetic. Once the client considers all this, she may simply choose to leave the tree untreated and deal with some less than perfect leaves.
Ultimately Certified Arborists that are licensed commercial applicators will be making applications based on label recommendations provided by the specific chemical manufacturer. However, we need to follow the thought process outlined above to make responsible decisions, understanding that our choice of which chemical to use when to apply it, and how to apply it may have effects that reach further than simply trying to save one plant, or make it look better.
José Fernández
ISA BOARD CERTIFIED MASTER ARBORIST® OH-5129B
ODA COMM. PESTICIDE LIC. #105859 (Categories 2B, 4A, 6A, Core)
TREE RISK ASSESSMENT QUALIFICATION (TRAQ)