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Student Research Spotlight - Hannah Stewart

Posted: July 7, 2017

This is the 11th of thirteen short news articles written by students, during the professional development class, about each other's research.

Fit or Fat: How insects handle parasitic infections
By Elizabeth Davidson-Lowe

What happens when bugs get fat? That’s one question Hannah Stewart, a PhD student in the Department of Entomology at Penn State University is trying to answer. Stewart studies host-parasite interactions and is focusing on a group of parasites called gregarines. Gregarines are tiny parasites that infect many insects and invertebrates, but although they are one of the oldest recorded parasites, we still know very little about how they affect insect health. Stewart’s job is to look at how gregarine infections impact insect behavior and physiology, and whether different types of insects react to gregarines in the same way.

“We know in dragonflies, when they are infected with gregarines, they don’t fly as well,” says Stewart, “We want to see how conserved this is so we will look at bumblebees and neogregarines [a parasite that is closely related to gregarines] to see if this is a conserved response”.

Although gregarines do not directly kill their hosts, their impact on behavior and flight performance can have lasting effects on insect development and success. For example, when dragonflies are infected with gregarines, fat accumulates near the flight muscles, potentially causing reduced flight performance, which can make it difficult to find food or mates.

In contrast, when bumblebees are infected they actually lose weight. “A bumblebee flies primarily by sugar,” Stewart explains, “And when they get a neogregarine infection, this replicates in their fat bodies. Bumblebees actually become skinnier when infected with a neogregarine. However, you see an increase in the amount of sugar they want, so it still most likely is affecting their flight”. Data suggests that when a bumblebee is infected, they discriminate less between food quality. If infected bumblebees are also poor fliers, they are less likely to seek only high sugar food sources and will accept food of any quality because they need to eat. This means that less food will make it back to the colony and the food will be of lower quality.

Stewart predicts that these parasites are manipulating host metabolic pathways to increase sugar availability. “A possibility here is that the gregarine/neogregarine says ‘I want sugar’ and pumps up that pathway so there is lots of sugar for them to steal from the host.” This may be the reason why gregarine infections in dragonflies lead to fat build-up and why bumblebees seek out any sugar source they can find. Even though these infections might be phenotypically different, they could lead to similar effects on insect health, albeit through different pathways.