Review: Girolami et al. (2009). Journal of Economic Entomology, 102: 1808-1815

Written: November 1, 2009

Posted: 11/02/09

Word count: 753

Question: Are neonicotinoid pesticides toxic to honey bees?

Answer: Yes, but that doesn’t mean they’re the cause of CCD

Practicing modern-day agriculture is centered around the balance between risk and reward. We have 6+ billion people on this planet that need to eat, exponential population growth, and dwindling natural resources and irrigable land. This places a high priority on maximizing efficiency in our farming systems, to get the most out of each acre. There are bounds, however, as we do not wish to sacrifice long-term sustainability for the sake or short-term gain. Therefore, we are forever searching for an optimum balance between food production and risk to the environment and human health. That proper balance, however, can be very subjective.

This impetus to balance risk with reward is responsible for the creation of a relatively new class of insecticides, called neonicotinoids. Based on certain anti-insect compounds from the tobacco plant, these chemicals can be incorporated into the entire plant treated with it, meaning that the insecticide is systemic in all of its tissues for a prolonged period of time. When an insect eats the plant, it ingests some of the pesticide, and it dies. These neonicotinoids are now used in many crops, golf courses, and even household pets to control fleas!

But these pesticides have been questioned as to their connection with insect pollinators, particularly with honey bees. They have been banned in certain European countries, and some have speculated that they may be responsible for Colony Collapse Disorder (CCD) here in the US. Empirical evidence, therefore, is desperately needed to make such a connection.

A recent report out of Italy looked at corn plants treated with these neonicotinoids. Corn plants can produce tiny droplets of dew on their leaves, called guttation drops, formed from the plant sap being excreted under certain conditions. Normally, of course, corn is wind-pollinated and not bee-pollinated, but it is possible that bees might forage for these guttation drops under certain conditions. The researchers sampled these droplets and fed them to adult worker honey bees. They found the droplets to be highly toxic, killing the bees within minutes of ingestion. They also fed bees with known doses of several neonicotinoid pesticides to generate mortality curves, demonstrating that all are highly toxic to bees are relatively low doses. Clearly, these pesticides are problematic for bees, and thus their usage needs to incorporate the impacts on pollinators.

However, it has always been known that neonicotinoids are highly toxic to bees. In fact, their label highlights this fact and specifically requires bees and other pollinators to be taken into account when they are used. The real question is what are the impacts of neonicotinoids, and guttation drops in corn, on honey bees in the environment. It is here that this study is more speculative than conclusive. The study didn’t measure any foraging rates on corn droplets, so it is unclear how much of a risk these high levels of pesticide might be. They also didn’t directly measure the neonicotinoids in the guttation drops using any chemical analyses, so there is no direct link to bee toxicity (that said, the compelling evidence is that the drops are toxic, even though the link is assumed more than empirically demonstrated). Finally, and perhaps most importantly, there was no measurement of the effect on colonies. Given that corn usually produces guttation drops in the spring (as highlighted by the authors), that CCD is probably a year-round phenomenon concentrated in the fall, and that other research has not found strong links between neonics or their by-products and CCD, there are many more links that need to be established to assess the true risk of these pesticides to bee health.

This study has stimulated a lot of discussion and additional research into the use of systemic pesticides and their impacts on honey bees. However, we need to place each piece of evidence into context and take caution in its interpretation. Clearly, these pesticides can be harmful to bees, that is nothing new. The question that needs to be answered, though, is whether or not the risk is worth it.

Reference

Girolami, V., L. Mazzon, A. Squartini, N. Mori, M. Marzaro, A. Di Bernardo, M. Greatti, C. Giorio, and A. Tapparo. (2009). Translocation of neonicotinoid insecticides from coated seeds to seedling guttation drops: a novel way of intoxication for bees. Journal of Economic Entomology, 102: 1808-1815.