From midsummer through fall, dazzlingly diverse dahlias (Dahlia spp.) fill gardens with flowers of button to dinner-plate dimensions, in every shade but blue and in color combinations too numerous to count. Grown as perennials in U.S. Department of Agriculture plant hardiness zones 7 through 10, and lifted for winter storage elsewhere, dahlias often attract two damaging black bugs: black bean aphids and thrips.
Black Bean Aphids
You may not notice black bean aphids (Aphis fabae) until they mass on your dahlia's new stems and leaves, but they've been hanging around for months. The current generation spent the winter and spring as eggs or wingless females on spindle trees (Euonymus spp), snowball bushes (Viburnum spp.) or mock orange bushes (Philadelphus spp.) before a generation of winged females migrated to dahlias and other summer hosts.
Black bean aphids drain sap and excrete the leftovers as gooey, transparent honeydew, which often attracts leaf-blackening sooty mold fungus. The females may give birth to five young each day, most of them also pregnant and capable of reproducing within a week. Even worse, the pests transmit several viruses, including leaf-mottling dahlia mosaic and cucumber mosaic virus.
At the first sign of infestation, set your hose spray attachment to jet or strong spray and shower the plant until all the aphids are dislodged. Do this in early morning so the leaves dry quickly and have less risk of fungal infection. Repeat daily until the aphids are gone.
Treat a large aphid population with insecticidal soap.
Water the dahlia well. Choose a time of day when the temperature is below 90 degrees Fahrenheit and the dahlia is out of direct sun.
Put on the protective clothing, safety goggles and waterproof gloves.
Spray the dahlia until the soap drips from all its surface**s**, making sure the insecticidal soap covers the stems and both sides of the leaves.
Repeat weekly, or as often as the insecticidal soap label recommends, until the infestation subsides.
Tiny, black greenhouse thrips (Heliothrips haemorrhoidalis) make a mess of dahlias, distorting and discoloring their flowers and blemishing their leaves with whitish scars and black waste.
Manage thrips as you would aphids:
- Dislodge small numbers with a forceful spray of water applied early in the morning for three consecutive days or until the insects are gone.
- Eliminate a heavy infestation by spraying with insecticidal soap every three days for two weeks.