Many insects can feed on various parts of the plum tree and threaten its life. Insects include eastern tent caterpillars, mealy bugs, peach tree borers, leaf curl plum aphids, and plum rust mites. Controlling these insects can involve manual, biological and chemical solutions.
Eastern Tent Caterpillars
The eastern tent caterpillar (Malacosoma americanum) spins white, silky nests resembling tents in the fork of tree branches. Tents house hundreds or thousands of caterpillars. These tiny black caterpillars have thick, tan hair. They sport a blue-and-white mottled pattern, with a white back stripe.
Egg masses in a shiny, dark foam bear 150 to 350 eggs. In spring, colonies feed on leaves in the morning or afternoon. Thousands of eastern tent caterpillars seek a new food source, then return to the tent. Their silk trails allow access for other larvae. Plum trees can survive by generating a new set of leaves. Controls include cutting off the tent and disposing in soapy water.
Mealy bugs, or woolly aphids, reach 1/10 to 1/2 inch in length and resemble puffy, white cotton. Their elliptical-shaped bodies feature short spines. Mealy bugs grasp stem leaves, growing tips and the fruit of plants. With their sucking mouthparts, they eat all parts of a plant, drink plant fluids and spread disease.
Symptoms include yellow leaves or leaf drop, and a honeydew-like substance over the plant surface. Pale yellow nymphs produce white, waxy coatings resembling cotton on their bodies. Males develop in a thin cocoon where they grow wings. Optimal growing time occurs during hot, dry weather and many generations can exist in one year. You can control mealy bugs with their natural predators, such as the mealy bug destroyer (bug destroyer (cryptolaemus or crypto-bug)), green lacewings, lady beetles or ladybirds. Other controls include physically removing and crushing the insect, or spraying insecticidal soap and/or applying horticultural oils.
Peach Tree Borers
The peach tree borer (Synanthedon exitiosa) especially threatens the Newport plum tree. This species damages by boring into the tree's bark, where infestations often go undetected. A moth can lay eggs on the tree trunk in August and September. Larvae bore tunnels that appear as round, oval or semicircular holes on the bark.
Symptoms include dying tree parts and noticeable external damage. A large amount of sap at the trunk's base indicates damage that can cause the tree to die. Prevention includes taking care that lawnmowers and weed trimmers do not injure the tree or give insects access through exposed tree parts. In cold weather, avoid pruning and thinning your plum trees.
Leaf Curl Plum Aphids
Leaf curl plums aphids (Brachycaudus helichrysi) reach less than 1/2 inch in length. This pear-shaped insect sports a pale-green to yellow colour in summer, then darkens to green or brown. Feeding on leaves results in curling. Ways to combat these include applying a low-nitrogen fertiliser, rather than a high-nitrogen one, and manually removing the insects.
Plum Rust Mites
Plum rust mites (PRM) (Aculus fockeui) appear yellow to pinkish-white or purple. Females overwinter on dead or shrunken buds, or crevices of bark and twigs. They migrate to feed on new leaves on spring buds. Feeding causes a toxaemia known as chlorotic fleck on plum. Symptoms include dwarfed, new leaves turning a silver or bronze and curling. The population of PRM peaks, then reduces drastically in hot weather.
If predator mites experience reduced numbers from a chemical spray programme, the PRM population could increase and cause problems. Controls can include introducing predacious mites and sulphur sprays.