A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

My marijuana plants have bugs! What do I do?

August 22, 1975

Dear Cecil:

I am sure that many of your readers would appreciate an answer to this most perplexing problem. All my marijuana plants are infested with tiny black bugs that suck out the juices, leaving them at first a speckled white, then yellow and worthless. Do you know of any non-toxic (to humans) spray or insecticide, preferably concocted from natural ingredients, that would get rid of them? Hurry, these mites are spreading to my other plants.

Dear Reader:

Hmm. Have you tried smoking them?

You don't describe the little buggers very completely, but my guess is that you've got aphids--some species of aphids are dark green or black, and sucking is their forte. 

The best "natural" deterrent to aphids is the ladybug. You'll find several suppliers listed in almost any issue of Organic Gardening. If this idea doesn't appeal to you, try cigarette butts. No kidding. Drop two or three cigarette butts in a quart of warm water, let sit overnight, strain, and spray the plants completely. Repeat after six days. Supposedly the butts contain nicotine sulfate, which kills any bug--mite, aphid, you name it. Several of Cecil's devoted readers swear by this method.

Alternatively, you can try 'most any house and garden spray containing pyrethrum (made from chrysanthemums) as the main active ingredient. It's safe as long as you use it according to the directions. Make sure you spray several times to make sure you nail each generation as it hatches.

If that doesn't work--Cecil is a great believer in contingency plans--you'll have to escalate to Diazinon, which breaks down (and is thus safe for eating and smoking, if you catch my drift) in three to four days. Malathion, the strontium-90 of aphid control, takes a week or more to break down and should be used only as a last resort.

FOLLOW-UP: SEZ WHO?

Dear Cecil:

An error appears in The Straight Dope. You recommend to a reader that he treat his marijuana plants with Diazinon as a last resort in order to rid them of your hastily-arrived-at diagnosis of an aphid infestation.

Based on his description, I would question your insect ID. But put that aside. Contrary to what you say, it is an established fact that Diazinon is a far more toxic compound than Malathion. Diazinon is described by Metcalf and Flint (Destructive and Useful Insects) as a persistent, general purpose insecticide with an oral LD50 of about 125 mg for rats and about 1000 mg administered dermally to rabbits. ["LD50" means the lethal dosage necessary to kill 50 percent of the target population. --C.A.]

Malathion is described by the same source as the safest of all insecticides with an oral LD50 of between 1400-5800 mg for rats. Supposing your correspondent weighed 72 kg (160 pounds), he would need to ingest approximately 216 grams of Malathion as compared to 9 grams of Diazinon to have a 50 percent chance of killing himself.  Your characterization of Malathion as "the strontium-90 of aphid control" is puzzling. There are widely used compounds that are far more toxic--Thiodan, Guthion, etcetera.

Lastly, you should know that by following your recommendation an individual would be in direct violation of federal law, which states that an applicant may not use any pesticide in a manner not permitted by the labeling. As the crop in question is not listed on the label, using it would be a violation which could result in civil penalties of as much as $5,000. Stick to calorie counting. --David D., Whiting, Indiana

Dear David:

If you got busted for growing 40 tons of hemp out in your back yard, I would venture to say that improper pesticide use would be the least of your problems. As for Malathion, my characterization admittedly was extreme, but not without support in the scientific literature.

Melvin D. Reuber, a former pathologist at the Frederick Cancer Research Center in Maryland, concluded in a 1985 article in Environmental Research that malathion was carcinogenic. The Environmental Protection Agency plans to request new studies on malathion-induced birth defects and chromosome aberrations from manufacturers of the pesticide in 1988. So there.

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