Pyrethrum

Marijuana Horticulture

by Jorge Cervantes

Ingredients – Pyrenthrum, the best known botanical pesticide, is extracted from the flowers of the pyrenthrum chrysanthemum, Chrysanth-emum coccineum and C. cinerariifoliu. Pyrenthrins – pyrenthrins, cinerins, and jamolins – are the active ingredients in natural pyrenthrun and kills insects on contact. Pyrethrum is often combined with rotenone or ryania to ensure effectiveness. Aerosol forms contain synergists.

Controls – A broad-spectrum contact pesticide, pyrenthrum kills aphids, whiteflies, spider mites, and insects including beneficials. It is very effective to control flying insects, but they must receive a killing knockdown dose, or they may revive and buzz off.

Caution – Do not mix with sulfur, lime, copper, or soaps. The high pH of these substances render it ineffective. Wash these substances off foliage with plain water (pH below 7) before applying pyrethrum.

Mixing – Mix in water with a pH below 7 and use a spreader-sticker.

Application – Spot spray infested plants. Aerosol sprays are most effective especially on spider mites. This can burn foliage – spray is ice-cold when it exits the nozzle – if applied closer than one foot. Aerosol spray contains a synergist, piperonyl butoxide (PBO) or MGK 264. Both are toxic to people. Pyrethrum dissipates within a few hours in the presence of air, HID light, and sunlight. Overcome this limitation by applying just before turning off the lights, the circulation, and vent fans for the night. One manufacturer, Whidmere, offers encapsulated pyrethrum in aerosol form called Exclude. As the spray fogs out of the nozzle, a bubble forms around each droplet of pyrethrum mist. The outside coating keeps the pyrethrum intact and extends its life for several days. When a pest prances by touching the bubble, it bursts, releasing the pyrethrum. Liquid and wettable pyrethrum applied with a pump type sprayer is difficult to apply under leaves where spider mites live.

Persistence – Effective several hours after application when the lights are on, longer when applied after light out and the fan is turned off.

Forms – Wettable powder, dust, liquid, granular bait, and aerosol.

Toxicity – Not toxic to animals and humans when eaten, but becomes toxic to people when inhaled. It is toxic to fish and beneficials.

Safety – Wear a mask and protective clothing when applying sprays or breathing in any form of pyrethrum, especially aerosols. Aerosols contain toxic PBO and MGK 464 – possible carcinogens – which are easily inhaled.