One group of pesticides that I have been devoting more and more time to in my Master Gardener training classes is the neonicotinoids. This group of pesticides received quite a bit of news this past week, as news outlets reported on two studies that link bees' declines with neonicotinoids.
Imidacloprid is perhaps the most common neonicotinoid pesticide available to home gardeners, as it was the first neonicotinoid to gain widespread use (Mullins 1993). Imidacloprid is a systemic insecticide. When applied as a soil drench or soil injection, it is absorbed by the plant's root system, and distributed to different plant tissues via the plant's vascular system. Imidacloprid can also be applied to plant leaves. This is known as a foliar application. In general, soil applications of imidacloprid take longer to work against insects, compared to foliar applications. However, soil applications of imidacloprid are effective for longer, compared to foliar applications.
There are two primary advantages of using a systemic insecticide, such as imidacloprid, against insect pests.
- First, imidacloprid is highly specific to insects. Imidacloprid is thus less likely to have negative effects on humans and other mammals, compared to other pesticides - such as the organophosphates or carbamates.
- Second, since soil applied imidacloprid is taken up by the plant's roots and incorporated into plant tissues - by definition - only a pest of that plant will get a dose of the insecticide. An insect has to feed on the plant to get a dose of the pesticide. Sitting on the plant is not enough to get a dose. This is one advantage of soil applied systemics, compared to insecticide sprays. With broad spectrum insecticide sprays, every insect in the path of the spray - whether they are the target pest, or a beneficial lacewing - gets a dose of the insecticide. Systemic pesticides diminish these 'non-target effects'. Of course, there is the issue of systemic insecticides being translocated to the nectar and pollen of plants. I'll cover this issue in a future post. Nonetheless, I'll mention here that one way landscape managers try to reduce the impacts of systemic insecticides on bees is by limiting their application to wind-pollinated trees - such as hemlocks.
- They are easier to apply to trees and large shrubs, relative to foliar sprays, and
- They offer long term protection against pests. In fact, you can often spot those pesticides that are likely to contain imidacloprid as an active ingredient, by looking for phrases such as '12 month control' or 'season long control' on the container.
- Szczepaniec and Raupp (2007) found that toxicity persisted, and pests were absent, up to three years following the application of imidacloprid to potted cotoneaster. It is important to note that the researchers did not sample foliage or look for pests in year four. They stopped their assays three years following the application of imidacloprid.
- Cowles and Lagalante (2009) found that a single soil injection to hemlock can provide between 5-7 years of pest protection, and that imidacloprid was detectable up to 8 years after application. After 5 years, the metabolite imidacloprid olefin was more abundant in tree tissue than imidacloprid. Although the authors of this report note that the olefin metabolite is much less toxic than imidacloprid, Nauen et al. (1999) report that the metabolites (including olefine) of imidacloprid retain toxicity to at least some insect pests.
- Dilling et al. (2010) found that trunk injection of imidacloprid in hemlock trees provided protection against hemlock wooly adelgid for 3-5 years.
- Webb et al. (2003) also found that a soil drench of imidacloprid protected hemlock trees from hemlock wooly adelgid for almost three years - the extent of the assays in this study.
Instead, I'm writing this post because I worry that too many home gardeners see the advertisement of '12 month control' on the front of the label, and think 'hmmm . . . . why purchase a product that can give me short term control, when I can get one with really long term control? That seems like the best buy!'
If you are having a short-term, one-time problem with aphids on your rose bushes, I'm not sure that imidacloprid is your best choice. In fact, for short-term, one time pest problems on any plant, I'm not so sure that imidacloprid is the best choice. Thus, I wanted to write a post that points out that the long term protection advertised may be RRREEEEAAAALLLLLlllllyyyyyy long term protection.
It's interesting to note that only one of the studies that I listed (Szczepaniec and Raupp (2007)) looked at the long-term persistence of imidicloprid in an ornamental plant. It's also interesting to note that many of these studies could detect imidacloprid (via bio-assay of pests or chemical assays for the active ingredient) up through their last sampling date. Thus, there is clearly more to learn about the persistence of imidacloprid in home gardens.
Are soil-applied systemic insecticides the best choice for pest problems in the home garden?
Are annual applications necessary?
I can't answer these questions - but I think they're important questions to ask.