Vanessa cardui
Pest description and crop damage Spiny, dark caterpillars with pale yellow stripes on sides. Larvae feed communally, associated with webbing and black frass.
Biology and life history Butterflies migrate into Oregon from California during springs following mild winters and lay eggs on thistle weeds in mint and other crops beginning in late spring. Larvae may migrate to and readily feed on leaves of mint and other crops or weeds in years of abundance. This insect is beneficial when feeding on thistle.
Scouting and thresholds Note that treatable field populations are rare. However economic injury may occur if larvae defoliate mint leaves from July through harvest and reach numbers given for cutworms and armyworms above.
Management-chemical control
Insecticides timed for looper or early season cutworm control will provide adequate control.