You are excited to be growing asparagus in your garden this year. Shortly after the spears come up, you notice that some plants are bent over. When you look closely you also see some small insects.
1. What are these insects?
They are common asparagus beetles. These beetles are slender, 1/4 inch long, have a metallic blue-black body with a reddish area behind the head and three pairs of yellow spots. They are first active from late April to early May (about when asparagus first comes up).
2. What kind of damage can they cause?
This asparagus beetle feeds on spears which may cause them to bend over into what is called a shepherd's crook. Their feeding also can cause browning and scarring. Common asparagus beetles can also lay a lot of eggs on the spears which can make asparagus unattractive. Both larvae and adults will feed on the ferns when they appear later in the spring. If the ferns are seriously defoliated, it can damage the plants.
3. How many of these insects can asparagus tolerate?
It is a good idea to check your asparagus regularly for common asparagus beetles as the spears come up. Asparagus beetles are most active in afternoon so check then to get an accurate idea of how numerous they are. If you don't find many insects or none at all, you don't need to treat your asparagus. If you find beetles on 10% or more of the spears, protect your plants to avoid injury. You should also treat your asparagus if 50% to 75% of the plants have common asparagus beetle larvae, or if you see two out of 10 spears with dark brown, oval-shaped eggs.
4. What are the best methods to manage them?
The best time for management is in early to mid May before too much damage has occurred. If your garden is small enough, try handpicking the asparagus beetles you seen. Just pick them off or knock them into a pail of soapy water. If this is not practical and you would like to spray the beetles, use a residual product that is labeled for asparagus, such as permethrin.
Once adults start laying eggs, a tiny (1/16 inch long) metallic green parasitic wasp can be found on the asparagus parasitizing beetle eggs. They can be quite effective in reducing the number of asparagus beetles in your garden. Don't spray when they're in your garden to help conserve them.
