The harlequin bug (Murgantia histrionica) is a brightly colored shield bug notorious for its destructive feeding habits in gardens. This pest targets plants primarily within the Brassicaceae family, including broccoli, cabbage, kale, and mustard. Both adults and nymphs are piercing-sucking insects that extract plant juices, causing localized damage. This feeding results in white or yellow blotches (stippling) on the leaves, leading to wilting, stunted growth, and eventual plant death under heavy infestation. Addressing an outbreak requires a tiered approach, starting with the least toxic methods.
Physical Removal Techniques
For small, localized infestations, direct physical removal offers immediate, non-toxic control safe for food crops. Adults and nymphs are easy to spot due to their distinctive black and red, orange, or yellow markings. Hand-picking the insects and dropping them into soapy water ensures they drown.
The eggs are laid in neat, double-row clusters on the undersides of leaves. These masses must be crushed or clipped off and destroyed to prevent hatching. For larger numbers, a handheld vacuum cleaner can quickly suck the insects from the plants. This works best in the early morning when bugs are sluggish, but the collection bag must be immediately sealed and disposed of or frozen to kill the pests.
Natural Sprays and Targeted Treatments
When physical removal is impractical, targeted treatments with natural compounds provide the next line of defense. Insecticidal soap, a solution of potassium salts of fatty acids, kills harlequin bugs on contact by disrupting their outer membrane. Since the product has no residual effect once dry, thorough coverage is necessary, and the spray must directly hit the pests, especially the nymphs.
Neem oil, derived from neem tree seeds, works as both a contact killer and a repellent. Its active ingredient, azadirachtin, disrupts the bug’s growth and reproductive cycles. Apply neem oil as a foliar spray, targeting both sides of leaves, timed for late evening to minimize harm to beneficial insects. Diatomaceous earth (DE) is a fine powder that kills insects by physical action, scratching the exoskeleton and causing dehydration. DE must be applied when plants are dry, dusted around the base and on the leaves, and requires reapplication after rain or overhead watering.
Synthetic Insecticides for Severe Infestations
Synthetic chemical controls are reserved as a last resort for outbreaks too large for cultural or natural spray methods. Due to the harlequin bug’s hard, shield-like shell, insecticides must be applied directly to the insect to be successful. Chemical formulations containing synthetic pyrethroids, such as bifenthrin or lambda-cyhalothrin, are commonly used for fast knockdown action against adults.
Carbaryl, a carbamate insecticide, is also effective against both adult bugs and nymphs. Before application, check the product label for the pre-harvest interval (PHI), which specifies the minimum days required between application and harvesting. Since these insecticides are broad-spectrum, they pose a significant threat to pollinators and beneficial insects. To mitigate this risk, avoid broadcast spraying and only apply the chemical when plants are not flowering, or apply late in the evening when pollinators are inactive.
Preventing Future Outbreaks
Long-term management focuses on cultural practices that deter the pest from colonizing the garden. Garden sanitation is paramount, involving removing and destroying crop residues and weeds in the fall, as adult harlequin bugs often overwinter in this debris. Eliminating these sheltered sites reduces the number of pests that emerge in the spring.
Lightweight floating row covers are an excellent physical barrier, preventing adult bugs from landing on susceptible plants. Install these covers immediately after planting and secure them tightly at the edges to prevent entry, removing them if the crop requires pollination. Trap cropping involves planting a preferred host, such as mustard or radishes, early in the season to lure the bugs away from the main crop. Once the bugs congregate on the trap crop, that section of plants can be treated or destroyed before the pests spread.