Numerous insect pests find dahlias delicious. In addition to the caterpillars and larvae, sucking insects are probably going to be the biggest issue with the most widespread offenders being: Aphids Thrips
Symptoms: wet, glistening, sticky film or fluid on leaves. Leave are curled, aphids are visible.
Diagnosis: Aphids. These are tiny insects that can easily be seen with the naked eye. They like to feed on fresh, young foliage and in doing so, they secrete a sticky substance that is known as honey dew. Aphids can come in many different colours.
Treatment: A strong jet of water from a hose will dislodge the pests. However, the plant may not be able to cope so a solution of soapy water may be applied. Alternatively, they can be picked off by hand. Many chemical pesticides are also available that can treat aphids.
Symptoms: the leaves start to change to a bronze colour with fine stippling on the upper surfaces. Very fine cobwebs can also appear within the leafy parts and the plant is starting to look unhealthy. Upon close inspection, tiny individual mites are just about visible to the naked eye.
Diagnosis: This is a spider mite infestation. Spider mites are less than 1mm in size and have two dark spots on their back. When they are active, they are a pale greenish brown colour, but this changes to a red colour in winter. The mites are highly destructive and work by sucking out the sap from the plant’s leaf cells. If you take a white piece of paper and shake the plant over it, tiny black dots will appear which indicates the presence of these undesirable pests.
Treatment: the mites can be easily dislodged by hosing the plants with a vigorous stream of water, taking care not to cause damage. Only very few products are capable of dealing with red spider mite, most insecticides will not be effective, so check labels carefully before purchasing.
THRIPS & THUNDERFLIES
Symptoms: leaves are distorted or not the right shape, plant looks unhealthy and tiny brown speckles are visible on lighter coloured blooms.
Diagnosis: this indicates an infestation of thrips or thunder flies. Adults lay their eggs on the leaves and when the young emerge, they feed on the flowers and buds. Adults also suck juices out of the leaves, causing them to become misshapen.
Treatment: there are a number of biological means of controlling thrips, mainly by introducing nematodes or soil based mites that feed on the thrips’ pupae. Proprietary chemicals are also available.
Symptoms: there is damage to the stem of the plant, with long pieces having been stripped away.
Diagnosis: This is indicative of wasp damage. Wasps strip the outer cuticle of the stem to use for nest building.
Treatment: this is very tricky to treat but hanging out traps that contain sugar solution amongst the stems can help