By Roger Marshall
Talk to any greenhouse grower and they will tell you that the two worst pest problems are aphids and whitefly. You might also find sow bugs, mealybugs, scale insects, and red spider mites. Larger greenhouse pests include slugs and snails that hitch a ride under the rim of large pots or in the drain holes of larger pots. Although screened windows can keep out butterflies and moths, you may also find a caterpillar or two feeding on your plants. Lastly, rodents can get into the greenhouse. Mice are a common problem and I’ve had groundhogs, skunks, and even a fisher that ate all the goldfish in my greenhouse pond one night when I forgot to close the door.
The best way to control insect pests is to be careful what you bring into your greenhouse. An isolation chamber can be a convenient way to keep new plants separate from the main population of greenhouse plants until you are satisfied that they are pest-free.
Whether to use insecticides is a personal decision. I prefer not to spray pesticides of any kind inside the greenhouse because they can kill off beneficial insects along with problematic ones and I find the effects of the product are multiplied by being in an enclosed structure. I have noticed that the odor of a pesticide in the greenhouse can last several days. Also, continual use of the same insecticide can eventually give rise to insecticide-resistant bugs.
My preference is to take the approach of Integrated Pest Management (ipm). This means I always begin with the least toxic type of prevention before moving on to more drastic measures. I start with barriers and screens to keep out pests, rotate crops, eliminate host plants in the vicinity of the greenhouse, isolate incoming plants, keep the greenhouse clean, handpick pests, and finally, bring in natural enemies such as ladybugs to feed on harmful insects. All these controls can easily be integrated into your greenhouse practice before you need to resort to chemical pesticides.