In his popular National Public Radio show, A Prairie Home Companion, creator Garrison Keillor spoke often of wonderful rhubarb pie in his comedy sketches. After a segment, he would often ask his audience, “Wouldn’t this be a great time for a piece of rhubarb pie?” Then he and his cast members would launch into a tune:
“Mama’s little baby loves rhubarb, rhubarb, Beebopareebop Rhubarb pie. Mama’s little baby loves rhubarb, rhubarb, Beebopareebop Rhubarb pie.”
Well, rhubarb growers certainly think everybody should love rhubarb in pies, cakes, breads or other tasty treats. To raise tasty rhubarb, growers must grow a quality crop, keeping it free of diseases, weeds and insects such as the rhubarb curculio, stem borer and others.
Entomologists though don’t believe insects are a major problem on rhubarb. Nonetheless, growers may see notches in the stem and leaf edges, sap released from wounds on the leaf stalks, and possibly decay if this insect feeds on the plant, according to information from entomologists in the Insect Diagnostic Laboratory at Cornell University.
A description of the rhubarb curculio isn’t pretty. It’s a beetle with a large, dark-colored snout, according to Cornell University. Growers will see a dusty powdery material on its back. Its head supports a curved snout with chewing mouth parts.
Entomologists at Cornell indicate that the insect’s eggs are oblong and a yellow to white color. The larva, once it matures, grows to about three-fourth inches long and has a brown head.
As an adult, the rhubarb curculio overwinters in debris or some protected area where rhubarb is planted, according to Cornell. By May, growers may notice this insect on the leaves and stalks. Quickly, they start laying eggs that are deposited in different cavities of about one-eighth inches deep in the stalks of host plants. In about a week to 10 days, hatching begins in those plants, but not the rhubarb plant. Then, the larva burrows into the stalk, reaching the bottom of the plant in a few weeks. Pupation occurs next at the base of host plants. Adults soon emerge, start feeding then look for a protected place for overwintering.
According to entomologists at the University of Illinois, the rhubarb curculio will not only bore into the stalks but also the crowns and roots.
To control the rhubarb curculio, growers can pick the beetles off plants in early summer if they don’t see too many, Cornell University entomologists recommend. In July, growers also can remove nearby hosts plants such as thistle, curly dock, sunflower and others.
The University of Illinois advises to definitely destroy curly dock, since it is a primary host. Growers also can treat the base of the rhubarb plant with an approved insecticide. Entomologists advise to burn infected rhubarb plants in July after most beetles have deposited their eggs.
Contact your local Extension agent for insecticide control options and rates.
Besides rhubarb curculio, the stem borer likes rhubarb too. This moth is brown to a reddish brown color, according to entomologists at Cornell University. It has expansive forewings, and hind wings appear a white to gray color and may sport a dark line down its center. Adults lay their eggs in groups between the leaf sheath and stem of perennial weedy grasses. Additionally, they lay these small, round and flattened eggs just above the soil surface. The eggs are a creamy color but may darken as the stem borer matures.
In late April and early May, Cornell entomologists indicate that larvae will hatch. The youngest larvae are a pinky to rosy color on their sides and back. As they age, larvae may lose their rosy color and turn a dirty white. The youngest larvae will feed on host stems for about two to three weeks. Once June arrives, they graduate from smaller stems to larger ones of hosts such as corn, potatoes and tomatoes. Larvae eventually leave host plants and pupate into the soil, and in about three weeks they come out as adult moths.
Rhubarb growers are warned that the stem borer can cause significant damage, destroying several plants in a row. The stem borer can tunnel into the stems, eventually causing leaves to wilt, turn brown and possibly die.
In addition to rhubarb, the stem borer enjoys the taste of corn, potatoes, tomatoes, pumpkins, beets and peas, according to Cornell University. Weeds are tasty for them too. They like quackgrass, green foxtail, barnyardgrass and curly dock.
The tachinid fly parasite may offer some control of stem borers, according to Cornell. However, entomologists suggest growers remove them by hand, cultivate or apply herbicides to any grassy weeds. Stem borers can’t lay their eggs on weed hosts. They also advise to keep borders and fencerows weed-free.
Contact your local Extension agent for insecticide control options and application rates.