The vegetable garden has a small bed of asparagus that’s been growing for 20+ years. Asparagus is a perennial vegetable that takes a little time to get established but once it’s growing, it’ll last for decades.
I’ve never had any problems with the asparagus aside from keeping the rabbits from eating the young stalks as they emerge from the ground. However this year I noticed something I’d never seen before. On some of the spears there were little black rods that were attached in vertical rows.
I wasn’t sure what those black rods were. I initially thought that they might be some kind of insect frass (the entomologist’s term for insect droppings) but the orderly way that they were attached to the spears made me doubt that I was seeing frass. Insect eggs seemed like a more likely identification.
One quick Google search identified these mysterious rods: they’re the eggs of the asparagus beetle (Crioceris asparagi). The adult beetle overwinters in debris near or in an asparagus bed and then emerges in the spring. The beetles feed on the emerging spears and lay their eggs on the spears in straight vertical rows. The cycle from egg to adult is a quick one: the eggs hatch in about a week; the larvae feed on the spears and ferns of asparagus, going through four instars in about eight days; the larvae then pupate in the soil and emerge as adults in another week.
In reading about this beetle, I learned that it’s most active in the afternoon. To this point I’d seen the eggs but I had yet to see an adult asparagus beetle. All of that changed when I went to the asparagus bed on a warm afternoon. There were adult beetles everywhere, chewing on the spears and mating away.
If I were a commercial grower of asparagus I’d have to do something to control these beetles. Their chewing can cause the spears to be twisted or deformed and the eggs would make the spears unmarketable. But as a home gardener, I can be a little less aggressive in dealing with this insect. I don’t care if the spears of asparagus aren’t perfectly formed. The eggs can easily be wiped off of the spears and if I miss a few, I just see it as a chance to add a little insect protein to the asparagus!
The presence of these insect did get me thinking about why I’m noticing them this year. I think it all has to do with cultural practices. In years past I would mulch the asparagus in the summer with straw and then in the spring burn the mulch and the dried ferns to clear the bed. I didn’t know it at the time but while clearing the bed with fire I was also killing most of the overwintering beetles.
With changes in the neighborhood I’ve replaced the spring burning with mulching the bed with wood chips and cutting the ferns off in the fall. What this means is that I’ve started to provide a perfect environment for the adult beetles to overwinter.
This fall I plan to remove most of the mulch from the beds. I might leave the ferns in the bed and go back to burning them in the spring. The other option is to cut the ferns in the fall and then go over the bed in the early spring with a propane weed torch. This would burn off the bottoms of the old fern stalks and kill many of the overwintering beetles.
In the meantime I’ll be keeping a close eye on the asparagus bed. Harvest season is coming to an end and once the ferns have fully developed I’ll be making sure that they’re not being defoliated by the beetles. This could weaken the plants and ruin next year’s harvest. If it gets too bad, a little pyrethrin spray will help to keep the beetles in check.
While asparagus beetles can be a problem, now that I know about them and their life cycle, I think I can get them under control. Cleaning the bed in the fall and using a little fire in the spring should be all that’s needed to limit the number of asparagus beetles in the garden next year.