Q. We have a pretty large greenhouse with several types of house plants inside. We've had problems with pests for as long as I can remember, mostly mealybugs and scale. The school does not allow us to use pesticides because of the students. We've been using dish soap, but it doesn't work. Any advice on safe ways to treat these nasty bugs?
---Gina; Lehigh Career and Technical Institute Floriculture Program; Schnecksville, PA
A. Well, first allow me to congratulate the school on their wise and admirable position. Because greenhouses are closed systems, workers are exposed to any toxins at a much higher concentration and for much longer periods of time than in outdoor agriculture. So it's very important for you and your fellow instructors to teach your students effective non-toxic methods of pest control. The positive cascade effect will be an enormous health benefit for future generations of workers in the "green industries".
Now, pest problems are always going to pop up, and greenhouse managers need to be able to recognize them early and act on them promptly. Prevention is also key. A professional greenhouse should always have some way to try and bar entry to hitchhiking bugs—like a fan that blows outward as workers and visitors come inside and/or those hanging plastic strips that give you a little 'wipe down' as you enter. New plants should also be cleaned thoroughly with sharp sprays of water before coming in, and ideally then reside for a while in a separate 'quarantine' section where they'll be closely inspected before joining the regular crew.
Our 'Common Sense Pest Control' experts at the BIRC—the Bio-Integral Resource Center in Berkeley, California —explain that mealybugs are a 'mobile scale' that, unlike the regular kind, can walk around on plants. (Regular scale exude a kind of natural glue that locks them in place.) Although they are individually very small—just 2 to 4 millimeters in length—mealybugs can appear in such huge numbers that it looks like the plants are covered in cotton. They excrete both a waxy material that coats their upper body and "honeydew", a ridiculously pleasant-sounding common name for their unpleasant poop, which is sweet and attracts ants.
(Note: The immature (larval) stage of the whitefly—another notorious greenhouse pest—looks so much like some mealybugs and scale that it was once classified as a member of that family. So if some of your mealybugs take to the air, you also have whitefly.)
With mealybugs, whitefly and/or aphids, the best plan of attack is to first organize a knockdown. On the next nice day, take all the plants outside and hose them down at close range with sharp streams of water, making sure you spray the undersides of the leaves well; that's where most of the mealybugs will be. You need a nozzle that delivers a really sharp spray to get results, but those results can be spectacular—you'll knock down 90% of the pests with water alone when you use it correctly and forcefully.
Once their numbers are diminished, decide on whether you'll use direct controls or beneficial insects to get the rest. It's no surprise that your "dish soap" didn't work; homemade soap sprays are often ineffective, and sometimes even harm plants. That's why both I and the BIRC strongly recommend buying professionally-made insecticidal soaps; they'll be much more effective. But you also have to use them correctly. Soap sprays work by smothering the insect in a soap bubble-like film, so you must spray and coat the actual pest; spraying a non-infested leaf just wastes good soap. It's also a good idea to rinse the plants down with plain water an hour or so later to protect them from any potential negative effects of the soap.
Ah, but the BIRC also reports that one 'home remedy' does work—swabbing mealybugs with a Q-Tip dipped in rubbing alcohol is lethal to the pests.
There's also a fascinating beneficial insect you can purchase and release in the greenhouse whose diet is so specific its common name is 'the mealybug destroyer'. It's an attractive member of the ladybug family that also feeds on scale, but you have to be careful not to use other controls after you release it, as the larval stage of the mealybug destroyer has a waxy coating that makes it look a lot like the pests its getting ready to devour. Both the adult and larval stage are beneficial—the babies suck the life out of young scale and mealybugs, and the adult beetles are powerful enough to rip the armor right off of an adult scale and then consume the tasty insides.
Green lacewings, perhaps the best all-around beneficial insect, also attack and consume mealybugs and scale. But the beautiful, fairy-like lacewing adults don't do this fine work; only their ferocious-looking alligator-like larvae dine on pests. So don't spray those scary-looking creatures; despite their monster-movie looks, they're the good guys. (And girls.)
And finally, both mealybugs and scale are highly attracted to plants that have been fed a lot of nitrogen, so don't feed greenhouse plants with chemical fertilizers—or even a lot of high-nitrogen natural plant food. Stick with things like compost, compost tea, worm castings and seaweed and kelp mixtures.