09
Aug
Insect “Honeydew” Secretions, Contaminated with Neonicotinoid Insecticides then Eaten by Other Insects, and Birds Contribute to an Expansive Threat
(Beyond Pesticides, August 9, 2019) A recent study, published in the Proceedings of the National Academy of Sciences, demonstrates a food chain vector for exposure of beneficial insects to neonicotinoid pesticides — the invasive mealybug, in this case. The finding may also be relevant for other phloem-feeding hemipterans, which can feed on neonicotinoid-contaminated plants and excrete so-called “honeydew” that is then consumed by beneficial insects. A primary “fix” for the decimation of insects and pollinators — caused chiefly by pesticide use, habitat destruction, and impacts of a rapidly changing climate — is, of course, the cessation of use of these toxic compounds in agriculture.
The most common route of exposure of beneficial insects to neonicotinoids is through contaminated floral nectar and pollen. The discovery of this “honeydew” vector is important because it could potentially affect far more insects than nectar and pollen consumption, given that honeydew is more abundant, especially in agricultural fields. Pollinators such as honey bees, solitary bees, bumblebees, and even birds have been observed feeding on honeydew.
Neonicotinoids represent more than 20% of the insecticides used worldwide; they are used on crops such as citrus, cotton, oilseed rape, soybean, fruits, potatoes, rice, corn, sunflowers (for seed), ornamentals, fruits, and greenhouse vegetables. The impacts of neonicotinoid (neonic) pesticide exposures on non-target organisms, and on various kinds of bees, in particular, have been well established and reviewed by Beyond Pesticides and many others.
Such exposures happen both directly, via application to plants and soils, and indirectly, through the food chain. The researchers, working out of Spain and The Netherlands, found that, “Neonicotinoids reach and kill beneficial insects when they feed on the most abundant carbohydrate source for insects in agroecosystems, honeydew. Honeydew is the excretion product of phloem-feeding hemipteran insects such as aphids, mealybugs, whiteflies, or psyllids.”
The study team applied two commonly used neonic insecticides, thiamethoxam and imidacloprid, on two batches of potted clementine trees. One batch received the insecticides via application to the soil (the most common application mode) at the recommended concentrations; to the other, the compounds were applied as a foliar spray, although at 50% of the recommended concentrations, in order to assess the effects when low doses of neonicotinoids reach honeydew producers. A third batch was “treated” with distilled water as a control.
Researchers then released small hordes of Planococcus citri, or the citrus mealybug, onto the trees to feed on them. Then, the beneficial insects Sphaerophoria rueppellii, or hoverfly (a pollinator in the adult stage and a predator in the juvenile stage) and Anagyrus pseudococci, a parasitic wasp, were fed with honeydew excreted by the mealybugs.
Results were bad news for beneficial hoverflies and parasitic wasps. Every hoverfly that ate honeydew from the thiamethoxam-sprayed trees died within three days of exposure, compared to 10% of the control group. Of the hoverflies that consumed honeydew from the trees soil-treated with thiamethoxam, nearly 70% died, compared with 14% for the controls. Results for the parasitic wasps were marginally better: more than 50% died after consuming honeydew from both soil- and foliar-treated trees, compared with less than 20% mortality among controls. The honeydew itself was also evaluated: samples from trees treated with thiamethoxam were highly toxic to both species of beneficial insects, and honeydew from those treated with imidacloprid was moderately toxic to hoverflies.
The researchers conclude that honeydew could be an important additional route of insecticide exposure for beneficial insects, including pollinators. They add that this vector of exposure could affect a much broader range of beneficial insects than contaminated nectar and pollen, and therefore, should be included in future environmental risk assessments.
This concern can now be added to the myriad threats contributing to the “insect apocalypse.” The role of industrial agriculture in creating these toxic risks to insects, never mind to human and environmental health broadly, is huge. In the U.S., in 2011 and 2012, 1.1 billion pounds of pesticides were used, primarily on crops; worldwide, the usage figure for the same period was nearly 6 billion pounds. (Tellingly, it’s difficult to find more-recent figures on use.) As Beyond Pesticides recently reported, “Multi-national agrichemical industries — companies like Bayer Monsanto, DowDupont, Syngenta, and the umbrella organization Croplife, that pervade our food system — share much of the blame. But through public pressure and consumer choice, we can shift towards alternative products and practices, improve biodiversity, and begin to repair the damage done by industrial agriculture.”
That shift — to organic and regenerative agriculture — must be a primary goal. Such a transition will benefit our pollinators, biodiversity at large, the health and functionality of natural environmental systems, human health, and even the ravages of a changing climate. The public can support the transition through consumer choices and public advocacy. Closer to home, learn about managing pests in non-agricultural environments without use of neonic insecticides. There are steps to take to support pollinators, including avoidance of toxic products; in 2013, Beyond Pesticides provided a list of such products.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Sources: https://www.pnas.org/content/early/2019/07/30/1904298116 and https://www.sciencemag.org/news/2019/08/tiny-insect-could-be-delivering-toxic-pesticides-honey-bees-and-other-beneficial-bugs
