Pesticides harm bees, but what about hoverflies?

Summer has come to an end, and with it the hot, sunny days that we all enjoyed. It was a good year for field work, as the uncharacteristically warm Irish summer made my job of sampling for my research easier.

I am a PhD student with a research focus on aphidophagous syrphids in cereal crops. Syrphids are commonly known as hoverflies, many of which have colour patterns that mimic bees and wasps. I focus on the species with larvae that feed on aphids (aphidophagous), therefore providing pest control services to tillage farmers (Figure 1). The adults, however, generally feed on pollen and nectar from flowers, providing pollination services like the bees and wasps they mimic.

Figure 1: Hoverfly eggs (white) and aphids on oat leaves. These will hatch into larvae that will feed on the aphids. Image Credit: Sarah Gabel

However, just as bees are facing threats of habitat degradation, so too are hoverflies. This summer, I was investigating the presence of one threat in particular – pesticides. Although intended to be applied to a certain aspect of the landscape (e.g. crops) to kill a certain kind of pest (e.g. aphids), pesticides can disperse throughout the environment, ending up in unintended places (e.g. wildflowers near a treated crop) and harming beneficial wildlife (e.g. pest-controlling, pollinating hoverflies). An adult hoverfly can inadvertently consume pesticide residues that ended up in the pollen and nectar of wildflowers. Likewise, hoverfly larvae can feed on aphids that had been feeding on treated crops, therefore being exposed to the pesticides the aphids were either sprayed with or ate themselves. To determine whether pesticides could be found in these food resources for hoverflies – and in what amounts – I sampled aphids from cereal crops, and pollen and nectar from blackberry flowers (Rubus fruticosus, Figure 2) in the hedges surrounding the crops.

Figure 2: A blackberry flower loaded with pollen. Image Credit: Sarah Gabel

With an amazing team of helpers, the samples were all collected, and are safely stored in a freezer, waiting for chemical analysis. Surveying of the adult fauna found a high presence of aphidophagous hoverflies, with the highest counts going to the iconic marmalade hoverfly (Episyrphus balteatus, Figure 3).  While there weren’t many hoverfly juveniles found hunting for aphids on the crops, there were plenty of other predators feeding on aphids – ladybirds, parasitoid wasps, and spiders just to name a few. Pesticide residues found in aphids could have an impact on all of these beneficial predators, just as residues found in the nectar and pollen of blackberry flowers could have an impact on all the pollinators feeding on them.

Figure 3: The marmalade hoverfly, Episyrphus balteatus. Image Credit: Sarah Gabel

While this work is mainly focused on the poisons in hoverfly food resources (aphids, nectar and pollen), the results will have significant meaning to all beneficial wildlife that depend on these resources and, subsequently, will have meaning to us humans as recipients of their ecosystem services.


Sarah Gabel (@SarahG10J) is a PhD student at Trinity College Dublin supervised by Professors Jane Stout and Blanaid White at DCU. She is studying aphidophagous hoverflies in cereal crops, and their interactions with the landscape. Sarah’s PhD is funded by the Irish Research Council (IRC).



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