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Dahlia - Pest Identification & Control:
Sometimes during the growing season the plants will be attacked by pests. The most common of these pests will be the cutworm, aphid, earwig, red spider mite, tarnished plant bug and snails & slugs. Cutworms are the most damaging to young Dahlias, chewing the plants at ground level. The best control is by hand-picking late at night, or by poison control. Aphids are sucking insects that multiply at an amazing rate. At the first sign of an infection, a regular spraying program should begin. A systemic insecticide will also keep a tight control on the problem. Common European Earwig is a nocturnal feeder that can be very damaging to Dahlias, especially late in the season when the flowers are developing. By chewing a hole in the developing flower bud, it will render the bloom worthless for showing. Also, once the flower has opened, earwigs will often take up residence in the rolled florets of the bloom itself. The first sign of this happening will be the dots of fecal matter on the blooms, a sure sign the critters are not far away. The old-fashioned method of trapping is still the best control. By using clay flower pots filled with Excelsior and placed on top of the supporting stakes, the pests can be trapped. The insects can be collected early in the morning and disposed of. Chemical treatment of the plant is often successful. Care must be taken with some earwig baits, if cats or other household pets are known to walk through the garden. What is toxic to earwigs may also be toxic to pets. Red Spider Mite usually has its best infestations during the hot dry summers. The mites live and breathe on the underside of the leaves where they suck the sap from the plant. The grayish-brown bleaching of the leaves show the presence of the mites in a web-like protective covering. This pest is best controlled by spraying the underside of the leaves with the appropriate chemicals, or by thoroughly hosing the undersides of the leaves. Tarnished Plant Bugs are small (about 1/4 inch long) brown-mottled, with shades of red and yellow. The bugs puncture the stems and buds causing distorted growth, with flower buds opening on one side, or not at all. Badly infested plants are short and never produce presentable blooms. Slugs are slimy 3/4 inch long snail type pests. Effective control of slugs is readily available by placing metaldehyde pellets on the soil around the plants. Other chemical controls may be found, but some are not to be used in areas where vegetables are grown, so be sure to read the label. Slugs need moist areas with protection from wind and sun, plus nourishment to grow and reproduce. By removing their shade, such as mulch, boards lying flat on the ground, and even weeds, and by close-cropping nearby grass, their habitat is minimized. They also dislike crawling over anything dry, dusty, or scratchy, so sand, coarse sawdust, cinders, etc., form a barrier to them. Essentially what happens, is that they have to do, is make a lot more mucus film so they can continue to move. With excessive production of this slime, they dehydrate themselves, and they perish. A circle of window screen around the plant, embedded into the soil can also be a barrier. Naturally, it is beneficial in these cases to trim the lower leaves, so the snails don't have an alternate path to dinner. Natural controls include toads and ducks. (You might need to have a rural property for the latter!) Grasshoppers are a challenge in some areas, usually near hayfields or open meadows, from mid August to near the end of September - in other words, during the peak blooming time for the Dahlia. See below for a picture of both insect and the extensive damage they can inflict. Control is next to impossible unless you have a garden which can be almost entirely screened in. The blooms have a profound attraction for these insects. While you must be lightning-quick to catch these bugs on the lawn, once they get focused on the bloom, you can often just bring you hand up over the edge of the flower (behind the grasshopper) and simply trap their legs between thumb and forefinger. It works 80% of the time. (Once you've caught them, I doubt I need to tell anyone what to do with them!)
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Send mail to
mg.thompson@sympatico.ca with
questions or comments
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