I get bugs eating on my rhubarb stems each year. It looks like eggs under the leaves, but I’ve never seen any bugs to identify. What are they and what can I do? Colorado potato beetles? Sevin?
Thanks for contacting Horticulture Talk through our Facebook page. To be honest, there are not a lot of insect pests that affect rhubarb — because it is acidic and because of the oxalate crystals in the leaves. However, based on your description, it sounds like you have Rhubarb Curculio (Lixus concavus).
The leaf stalks of the rhubarb may show exuding sap and partial decay from late-May through early summer, due to the feeding and egg laying punctures of the rhubarb curculio. Feeding injury appears as notches in the stem and on the leaf edges. Sap exudes from wounds of either type and collects as glistening drops of gum when fresh. Fortunately, the eggs of this insect do not hatch when deposited in rhubarb.
The rhubarb curculio (or rhubarb weevil) is a large snout beetle, about 1/2 inch long. It is dark colored, with a yellow powdery material dusted on its back. The yellowish covering easily rubs off when the insect is handled. The head has a downwardly curved snout, at the end of which are the mandibles (the chewing mouth parts). The eggs are oblong and yellow- white in color (similar to Colorado Potato Beetles). The mature larva is a legless grub about 3/4 inch in length, with a brown head.
The curculio overwinters as an adult, in piles of debris or in other protected places near the rhubarb planting. In about mid-May the adults appear, and are seen resting on the stalks and leaves of rhubarb, dock, thistle or sunflower. They soon begin laying eggs. Eggs are deposited singly in cavities about 1/8 inch deep in the stalks of host plants, and hatching occurs in a week to ten days, in all plants but the rhubarb. The rhubarb curculio survives in weeds in or near the garden. Eggs deposited in rhubarb do not hatch, but are killed by the actively growing plant tissue, which crushes them. In other hosts the newly hatched larva begins burrowing its way down through the stalk, so that when it reaches maturity in eight to nine weeks, it has reached the bottom of the stalk just below the soil surface. Usually one grub reaches maturity in a host plant. Pupation occurs in a cavity at the base of the host plant, and within a few weeks the adult beetles emerge. The adults feed for a short time, and then seek out protected places to spend the winter. There is only one generation of this insect a year.
The only direct method of control is to hand pick the beetles from the plants during early summer and destroy them. When the beetles first emerge they are easily picked from the vegetation on which they are resting. Their large size aids in finding them and helps make them easy to handle. The removal of all wild plants in which the beetles breed (dock, thistle, and sunflower) growing in or near the planting during July, while the curculio larvae are still in them, will also be helpful.
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