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Asparagus Beetles: Where are They Overwintering?

By Jennifer Zavalnitskaya and Zsofia Szendrei
Figure 1. Asparagus beetles lay eggs on spears during harvest (A), which reduces spear quality. These eggs do not develop into larvae during harvest, as spears are picked before they can hatch. After harvest during the fern stage, larvae (B) hatch from eggs, eat damage the fern and develop into adult beetles (C). There are typically two or three generations of asparagus beetles in Michigan with the last generation of adults seeking out overwintering shelter to survive the winter. They emerge in the spring to feed, mate and lay eggs. Photos by Jennifer Zavalnitskaya and Zsofia Szendrei, MSU Entomology.
Michigan’s asparagus industry has decades of experience managing the common asparagus beetle, but more recently this pest has been causing surprisingly difficult problems. On some well-managed farms, growers are having issues with asparagus beetle eggs on spears (Figure 1). These growers are making frequent insecticide applications to kill the beetles, but populations rebound quickly afterwards. Where exactly are these beetles coming from? In order to answer this question, it’s important to understand what these beetles are doing in the fall and throughout the upcoming winter.
Although it’s getting cooler during the fall, asparagus beetles are still active and seeking overwintering habitat all throughout and nearby Michigan asparagus fields. These are the same beetles that will emerge from overwintering when spring comes. The Vegetable Entomology Lab at Michigan State University is working with Michigan asparagus growers to discover where these beetles choose to overwinter. During winter farm visits, we checked tree bark, leaf litter and asparagus stalks to answer this question. We found beetles overwintering in many places, including decomposing asparagus stalks, under leaf litter and underneath tree bark (Figure 2). But just how many beetles are sheltering in these different places?
Figure 2. The MSU Vegetable Entomology Lab is investigating where asparagus beetles overwinter. We found them hiding within fields in decaying asparagus stalks (A) and outside fields under tree bark (B). In an experiment, we are placing adult beetles in overwintering cages with different types of bark, pine needles, leaves and asparagus stalks to see how these different shelters help beetles survive the winter cold (C). Photos by Jennifer Zavalnitskaya, MSU Entomology.
Based on data from winter 2019-2020, we found that some beetles overwinter in woodlots and weed margins around asparagus fields, which lines up with grower observations. We also found that a higher proportion of beetles overwinter within asparagus fields. So what’s happening within fields may be just as important as the types of trees that are nearby. How well do beetles survive in these different habitats?
Asparagus beetles undergo a physiological process to prepare their bodies to withstand freezing temperatures during winter. Temperatures can only get so low before beetles begin to die so beetles seek out shelter so they are protected as much as possible from freezing temperatures. Access to the right shelter could increase their survival into spring. We found that asparagus beetles that overwinter in deciduous leaves or old asparagus stalks survive at higher rates compared to pine needles and bark. 
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