Jesica Conrad/Master Gardener: Asparagus time means it’s beetle time, too
At long last, we can see the dirt and grass.
However, we need to be aware of insects that can interfere with our crop.
There are two types of asparagus beetles: Crioceris asperigi, the common asparagus beetle and Crioceris duodecimpunctata, the spotted asparagus beetle. It is good to be able to identify them as the common asparagus beetle is the more pesky. The common asparagus beetle adult is bluish-black with six cream colored spots on its back. One could confuse the beneficial lady beetles with the spotted beetles. They are both reddish-orange but the spotted asparagus beetle has 12 black spots and a distinct head; adult lady beetles have oval to almost round bodies and a varying number of spots. Their heads are partially concealed and have short antennae.
Common beetles overwinter in sheltered locations like the hollow stem of old asparagus plants.
Just as soon as spears are emerging so do these common beetles. Look for dark brown oval-shaped eggs on the ends of the spears. Hatching within a week, the grayish larvae move on to the ferns and munch happily away. The spotted beetles are similar but their eggs are green,the larvae are orange and they tend to feed on the berries of the asparagus. Appearing in mid-May, they are gone in July.
The damage causes browning, scarring and can cause the spear to assume a shepherd’s hook appearance. Plus, lots of eggs on the spears makes the vegetable unappetizing. If sufficient infestation occurs, the common beetles can devour the foliage. This can weaken the plants. The ferns feed the roots for the next year’s growth.
Sleuthing the plants in early May or just after the plants emerge is essential and continued sleuthing during the rest of the growing season is preferable in the afternoon when the beetles are most active. If you have a small garden, handpicking can be effective. Drop larvae and adults in a pan of soapy water. A large patch may need more ammunition. Consult the University of Minnesota Extension website for more detailed information about pesticides. If your patch requires sprays, then be mindful of bees and other pollinators. Bees do not pollinate asparagus but have been known to visit. Spray early in the morning or in the evening as the bees are less likely to be hanging out gathering nectar or pollen in the garden. An insecticide containing carbaryl is very harmful to bees.
There is a tiny green metallic wasp, Tetrastichus asparagus, that parasitizes asparagus beetle eggs. These have been known to provide very effective control. Watch for them and consider yourself lucky.
Clean up your patch in fall to eliminate plant residue which overwintering adults find desirable.
If you should use insecticides, please read the label closely to ensure proper application. A list of appropriate insecticides can be found at the University Of Minnesota Extension Service website, www.extension.umn.edu/garden/yard-garden under insects on vegetables and for more information on horticultural topics.
Local Master Gardeners will again answer your questions on home horticulture. Call (218) 444-7916, leave your name, number, and question and you will receive a return call.