Authors: Marissa Schuh, IPM Extension Educator, mschuh@umn.edu and Annie Klodd. Originally published 2022, updated in 2023.
Despite the 90 degree temperatures the southern portions of the state are experiencing, it is still spring, and spring means asparagus, which oftentimes means asparagus beetles.
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Asparagus beetles feed on spears and ferns. Photo: Whitney Cranshaw, Colorado State University, Bugwood.com |
First Question: Do I actually need to spray?
The threshold for asparagus beetles are:
5-10% of plants are infested with adults or larvae
2% of spears have eggs.
Asparagus beetle eggs are laid single-file in a line. Photo: Annie Klodd, UMN Extension.
Second question: What do I spray?
Using insecticides in asparagus is a challenge because of the constant picking. Any product that you do use needs to have a short Pre-Harvest Interval (PHI). This varies with each product. For example, Entrust (active ingredient: spinosad) has a 60 day PHI, meaning it could be used on asparagus beetles after you’re done picking, but has to be left in the shed until later this summer.
The Midwest Vegetable Production Guide can be used to quickly find options that work in asparagus beetles and have a shorter PHI. Options with shorter PHIs include:
There are organic products with asparagus beetles on their label, though we don’t have good data on their efficacy in our region. These include Pyganic (pyrethrins) and neem.
If asparagus beetles are a recurring problem, it is also important to check the label for maximum application limits. For example, Sevin XLR Plus has an application rate of 1-2 qts. per acre, with the label stating that you cannot use more than 5qts per year or more than 5 applications to the same asparagus spears/ferns in a year. Again, the Midwest Vegetable Production Guide outlines this information in a concise way.
Finally, it is hot out! This increases the risk of plant damage and drift, adjust your sprayer and consults the label for more information.
Third question: How do I get the most out of my spray?
Unfortunately, asparagus beetles are most active on warm sunny afternoons. This means the times when we want to be out picking are also the best window of time to make pesticide applications. If you are seeing significant damage, sacrifice a picking to ensure that the most beetles possible are contacting your spray.
Fourth question: Do I have to keep spraying as new spears emerge?
Two tricky things about spraying asparagus beetles are that 1) new spears are constantly emerging, and 2) asparagus beetles remain active the entire season, except for a brief period in June. New spears are not protected by insecticides that were sprayed prior to their emergence. After applying your first spray, scout the crop regularly to see if asparagus beetles or eggs appear again. If infestation reaches the action threshold (5-10% of spears are infested, or 2% of spears have eggs), it is time to spray again. Rotate insecticide modes of action to stay under the maximum allowable rates for each insecticide.
I’m busy now, can I do stuff later this year to prevent next year’s asparagus beetle?
Once we are done picking and letting a patch fern out, asparagus beetle is still hanging around. They continue to feed on ferns and build their populations, and research suggests a majority of asparagus beetles overwinter in fields. Once ferns are totally brown, remove them in the fall/early winter instead of waiting until spring.
If you are seeing high numbers of beetles in ferns later this summer, you could treat them. A threshold is 50-75% of ferns infested. Tread carefully, as flowering asparagus ferns are attractive to bees, so these applications have to be timed carefully.
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