The University of Nebraska has a short but succinct suggestion for troublesome Insects. Here is an excerpt form their website: http://ianrpubs.unl.edu/horticulture/g189.htm

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 (Di-
Syston) 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.