Orange and black harlequin bugs are a nightmare for cole crops.
In keeping with the fast-approaching Halloween holiday, the next few weeks we will visit bugs dressed in the Halloween colors of orange and black. Some will be spooky and noxious and others will be helpful and heroic. This week we go spooky and meet the ravager of radishes, killer of kale, and butcher of Brussels sprouts known as the harlequin bug. On a trip to a local community garden, a distraught gardener spoke to me about his withering kale crop. As I strolled around the garden, I spied kale, radishes, and cabbages stunted, discolored, and so severely misshapen that their only remaining use was to grace a compost heap. Upon closer inspection, I discovered legions of orange and black bugs and ranks of minute eggs two and three abreast on leaves. These rascals were harlequin bugs and their spawn. These common pests from the south are kin to several other true bugs we’ve met in previous episodes of Bug of the Week including squash bugs, boxelder bugs, red-shouldered bugs, brown marmorated stink bugs, and wheel bugs. What makes this bug a “true bug” is its gradual metamorphosis including egg, nymph, and adult life stages, sucking mouthparts, and wings partially membranous and partially leathery.
Distinctive barrel shaped eggs colored black and white are an excellent clue that harlequin bugs are about.
Harlequin bugs spend the winter earthbound, hiding in debris left behind from last year’s crops. With the return of spring and the rampant sprouting of wild mustard plants and early season cole crops in gardens, 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, Brussels sprouts, turnip, kale, horseradish, radish, and cabbage. Harlequin bugs also eat plants in the nightshade family such as eggplant. 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 generations have time to develop.
These tiny white barrels with black rings are eggs of harlequin bugs. After hatching, small wingless orange and black nymphs search leaves and eventually probe stems and foliage with beaks to remove nutrients. Prior to feeding, winged adults may do a little grooming before settling down to feed with their cronies. After a season of harlequin bug terror, kale and other cole crops may be a mess.
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 milkweed bugs 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 choose 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.
Feeding by wingless harlequin bug nymphs severely damages vegetables like cabbage, kale, broccoli, and other members of the mustard family.
Gardeners have a challenge to keep the buggers at bay and hours can be spent removing 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 at the end of the growing season and thermally composting it may further limit the number of tricksters surviving the winter in or near your garden. Harlequin bugs complete many generations in the tropics and Deep South and part of the reason we may be having more problems with these buggers locally resides in our generally warmer winters. A clever recent study found that when winter’s chill dips into the low teens and single digits, mortality of overwintering harlequin bugs increases dramatically. Perhaps true winter will return at the end of 2023 and in the New Year to bring some relief from these orange and black ghouls of the garden.
References
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 and Shetlar were used as 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, and the interesting article “Supercooling Points of Murgantia histrionica (Hemiptera: Pentatomidae) and Field Mortality in the Mid-Atlantic United States Following Lethal Low Temperatures” by A. S. DiMeglio, A. K. Wallingford, D. C. Weber, T. P. Kuhar, and D. Mullins was also used in preparation of this episode. To learn more about harlequin bugs and their management, please visit the following website: https://extension.umd.edu/resource/harlequin-bug-vegetables