Plum trees are popular for their fruit but susceptible to insect pests impacting health and production. Understanding common pests and management strategies is crucial for a healthy harvest.
Common Plum Tree Pests
Plum trees host various insect pests, each with distinct appearances and damaging behaviors.
Plum Curculio
Plum curculio ( Conotrachelus nenuphar ) is a small, dark brown or grayish-black weevil, about 1/5 inch long, with a rough back and snout. They emerge in spring, laying eggs in developing fruit and creating a crescent-shaped scar. Larvae, legless C-shaped grubs, burrow into fruit, causing rot and premature drop. Infested fruit may show oozing sap.
Aphids
Aphids are small, soft-bodied insects (less than 1/8 inch long), green, yellow, white, or brown. They cluster on leaf undersides and new shoots, sucking sap. Their feeding causes leaves to curl, distort, stunt growth, tree weakening, and reduced fruit quality. They also excrete honeydew, a sticky substance leading to sooty mold on leaves and fruit. Common types include plum leaf-curling and mealy plum aphids.
Borers (Peach Tree Borer & American Plum Borer)
Borers are moth or beetle larvae tunneling into plum tree bark and wood. Peach tree borer ( Synanthedon exitiosa ) larvae feed on the cambium layer, usually at the soil line or lower trunk. Signs include oozing sap mixed with sawdust-like frass at the tree’s base, and sometimes canopy dieback.
American plum borer ( Euzophera semifuneralis ) is another borer, with adult moths having gray forewings marked with brown and black. Its larvae also bore into the cambium layer, often around scaffold crotches and graft unions, leaving reddish-orange frass and gum pockets. This internal feeding can girdle branches or the trunk, leading to breakage or tree death, especially for young trees.
Spider Mites
Spider mites are tiny arachnids (less than 1/50 inch long), yellowish, green, red, or brown. They feed on leaf undersides by piercing plant cells and sucking out contents. This feeding causes stippling (tiny yellow or white dots), eventually bronzing or silvering leaves. Heavy infestations cause premature leaf drop and reduced tree vigor; fine webbing may be visible.
Scale Insects
Scale insects appear as small, immobile bumps on bark, twigs, and sometimes fruit. These sap-sucking pests secrete a waxy protective shell, making them difficult to spot. Infested branches may appear encrusted with circular, often gray or black, armored scales. Heavy infestations weaken the tree, leading to yellowing leaves, leaf drop, and stunted growth. The San Jose scale is a common type.
Preventative Measures for Plum Tree Bugs
Proactive measures reduce plum tree pest infestations by promoting tree health and creating a less hospitable environment.
Optimal tree health is a primary defense, making vigorous trees more resilient. This involves proper watering (especially during dry periods) and balanced fertilization to avoid excessive nitrogen, which encourages aphid and spider mite reproduction. Regular pruning enhances air circulation, deterring pests and reducing humidity.
Orchard sanitation prevents pests by removing overwintering sites and breeding grounds. Promptly collecting and destroying fallen fruit eliminates plum curculio larvae and reduces other pest sources. Clearing debris around the tree’s base also removes insect hiding places and eggs, disrupting pest life cycles.
Encouraging beneficial insects (e.g., ladybugs, lacewings, parasitic wasps) provides natural pest control. Planting flowering plants near plum trees attracts these predators, which feed on common pests like aphids and spider mites. Minimizing broad-spectrum insecticides preserves these helpful insect populations.
Dormant oil sprays in late winter or early spring are effective preventative measures. This horticultural oil suffocates overwintering eggs and adult pests like aphids, scale insects, and spider mites on bark before buds emerge. Apply dormant oil when temperatures are above 40°F and no freezing temperatures are expected for several days to avoid harming the tree.
Physical barriers offer direct protection against certain pests. Netting can cover trees, creating an exclusion zone against larger insects like plum curculio or birds. For borers, wrapping the lower trunk with a barrier can prevent egg-laying and larval entry.
Treating Plum Tree Bug Infestations
When pest infestations occur, reactive measures can control them.
Manual removal is effective for small pest populations. For aphids, a strong stream of water dislodges them. Hand-picking larger pests like plum curculio or shaking branches over a tarp in the early morning to collect dislodged beetles is also effective. Dispose of collected debris away from the orchard.
Organic sprays offer less toxic pest control. Neem oil, from the neem tree, acts as an insect growth regulator and repellent, disrupting pest feeding and reproduction. It is effective against aphids, spider mites, and some borers.
Insecticidal soaps dissolve the waxy coating of soft-bodied insects, leading to dehydration. These soaps are useful for aphids and spider mites and have low toxicity to beneficial insects once dry. Pyrethrin-based sprays, from chrysanthemums, offer a natural insecticide option for various pests, though they can affect beneficial insects.
Chemical insecticides provide aggressive control for severe infestations. Products containing spinosad are effective against pests like twig borers while being less harmful to beneficial organisms. Other options like permethrin control a broad spectrum of pests. Always follow label instructions regarding application rates, timing, and safety precautions. Apply these treatments at the correct developmental stage of the pest for maximum effectiveness and to minimize impact on non-target species.
Cultural control methods are also reactive. Pruning infested branches removes localized pest populations, such as those affected by scale insects or borers. Removing and destroying these pruned sections prevents further spread. Integrated Pest Management (IPM) combines cultural, biological, and chemical strategies. IPM emphasizes monitoring pest populations, using preventative measures, and applying targeted treatments only when necessary for long-term, sustainable pest control with minimal environmental impact.