Here’s some information about common pest problems and how to manage them with pesticides from an article on growing dahlias from the University of Nebraska – Lincoln which you can find here:
If you’re looking for a more natural approach, they also have a good site for information about organic pest management – https://agronomy.unl.edu/farming-systems/organic-pest-management.
Insect problems vary considerably from garden to garden and from area to area within the state. Examine dahlias weekly for evidence of damaging insects; be sure to inspect the undersides of the leaves. Use insecticides when insects and damage are present, and always follow label directions and precautions. Some of the more common insects you might encounter are:
- Aphids — Aphids are small, soft-bodied, sucking insects that congregate on the undersides of leaves or on the tips of new growth. When abundant they cause yellowing and curling of leaves. The most serious damage occurs when aphids transmit mosaic from infected plants to healthy plants. Foliar applications of malathion or diazinon should adequately control aphids. Applying the granular systemic insecticide, disulfoton (Di-Syston two percent granules), to the soil
provides extended control (six to eight weeks) without the necessity of frequent applications. Di-Syston also is effective against other sucking insects.
- Leafhoppers — The potato leafhopper is a frequent pest of dahlias. Leafhoppers, which are usually found on the undersides of the leaves, average about 1/4 inch long, are yellowish-green in both the adult and immature stages, and have a curious habit of running sideways. Potato leafhopper injury results in severely stunted plants that may not bloom. Early symptoms are pale-colored foliage that curls and browns along the leaf edges. If the leafhoppers are present,
malathion or diazinon will control them effectively, or disulfoton (DiSyston) can be used as a soil systemic.
- Plant Bugs — The four-lined plant bug and the tarnished plant bug occasionally damage dahlias. Of the two, the tarnished plant bug is the more common. This brownish, triangular-shaped insect feeds on the buds, resulting in one-sided flowers. Malathion or rotenone will control them, but frequent applications may be necessary if insects are abundant. Treat only if the insects and their damage are present.
- Spider Mites — Mites are tiny, and difficult to see with the unaided eye. Examine the undersides of leaves with a magnifying lens to detect mites before they reach damaging numbers, or briskly tap foliage over a white piece of paper; mites will drop to the paper and can be seen as small moving specks. When mites are abundant, fine webbing will be visible on the foliage; leaves will stunt, curl and turn bronze. Although seldom a problem, mites can be difficult to control
once established. Select a specific miticide such as dicofol (Kelthane) and make two applications seven to 10 days apart, thoroughly covering the undersides of the leaves each time.
- Stalk Borer — Stalk borers overwinter as eggs on weeds and tall common grasses. Upon hatching in spring, the small worms migrate to dahlias and bore into the lower stem. Keep the areas near the garden free of weeds and grasses, and remove or thoroughly cover plant material remaining in the garden in the fall. In severe problem areas it may be necessary to apply an insecticide to the lower stem and around the plant base weekly during June. Carbaryl (Sevin) is relatively
effective for stalk borer control.
- Thrips — Thrips are found in open flowers but also may damage opening buds if the insects are abundant. They are likely to be most troublesome if the weather is hot and dry. Injured areas turn white and then dry up. Buds become distorted. If thrips are present in damaging numbers, remove and destroy all open blooms; then treat remaining buds with malathion or diazinon.
Products listed in this publication are for the convenience of the reader, and are not intended as an endorsement. Use all chemicals according to manufacturer’s recommendations, and follow all safety precautions to prevent pollution, or damage to humans, plants and animals.
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