Last week a distraught gardener presented us with a very unhappy specimen of cabbage. This was not a candidate for slaw or stew, but one of those remarkably large cabbages widely used to decorate landscapes in autumn. The poor cabbage was stunted, discolored, and so severely misshapen that its only remaining use was to grace a compost heap. Upon closer inspection, we discovered ranks of minute eggs two and three abreast in rows on the leaves. The eggs resembled miniature beer kegs painted black and white. The eggs were a dead give away that harlequin bugs were at work. Harlequin bugs are one of the most colorful pests found in our gardens. They are kin to several other true bugs we met in previous episodes of Bug of the Week including honeylocust plant bugs, boxelder bugs, brown marmorated stink bugs, and wheel bugs.
What makes this bug “true” are its gradual metamorphosis including egg, nymph, and adult life stages and sucking mouthparts. Harlequin bugs spend the winter on the ground in debris left behind from last year’s crops. With the return of spring and the rampant sprouting of wild mustard plants, the survivors have a renewed source of food. Cuisine favored by harlequin bugs includes common weeds in the mustard family such as yellow rocket, black mustard, and peppergrass and several cultivated crops like broccoli, turnip, kale, horseradish, radish, and cabbage. Harlequin bugs also eat plants in the nightshade family such as egg plant. After feeding for several days, females deposit eggs on the surface of the leaf. The eggs hatch into brightly colored nymphs with powerful beaks used to probe tender tissues of the plant and extract nutrients. All of this probing and poking destroys cells and vascular tissues and robs the plant of nutrients needed for growth and development. After five nymphal stages, the bug transforms into an adult that continues to wage war on cabbage and other plants.
In southern climes, several generations can occur each year while in the north only one or two have time to develop. The most striking feature of these bugs is, without question, their remarkable coloration. In previous episodes we learned that bright colors of monarch butterflies and dogbane beetles served as a warning to would-be predators that these insects packed a potent chemical punch. The same is true for harlequin bugs. In a series of clever studies, Drs. Aliabadi, Renwick, and Whitman demonstrated that several species of predatory birds found harlequin bugs distasteful. Harlequin bugs remove noxious compounds called mustard oils from cabbage and other related plants in the mustard family. These noxious chemicals are stored in the body of the harlequin bug to give predators a nasty surprise if they chose to attack. After a few attempts to eat these spicy bugs, birds likely learn to seek more delectable meals elsewhere. The bright coloration provides a reminder of what they should not attempt to eat. Gardeners have a challenge to keep the buggers at bay and hours can be spent removing the adults and nymphs by hand. Another approach to foiling their plans is to plant a trap crop of cabbage or kale very early or very late in the season. In spring as survivors move to the early planted greens, wage war on the concentrated encampment of bugs to reduce the number left to plague crops later in the year. In fall, after most of your vegetables have been harvested, leave just a few cabbages behind to attract any harlequins lingering about. The bugs concentrate on the cabbages and can be annihilated, which reduces the number of bugs moving to the ground to spend the winter. Removing plant debris may further limit the number of tricksters completing the winter in your garden.
We thank Ethel Dutky for providing the cabbage and inspiration for this bug of the week. Two great reference books “Insects of Farm, Garden, and Orchard” by Davidson and Lyon, Wiley Press and “Garden Insects of North America: The Ultimate Guide to Backyard Bugs” by Cranshaw, Princeton Press were used as a sources of information for this Bug of the Week. Information on the chemical defenses of harlequin bugs was found in the article “Sequestration of Glucosinolates by Harlequin Bug Murgantia histrionica” by Aliabadi, Renwick, and Whitman, Journal of Chemical Ecology. For more information on these interesting bugs, please visit the following web site.