Aphids on a plum tree are a common issue for fruit growers, as these small, sap-sucking insects quickly undermine the tree’s health and productivity. Aphids weaken new growth and excrete a sticky substance called honeydew, which fosters the growth of unsightly sooty mold over leaves and fruit. Addressing an aphid infestation promptly is important because a heavy population can reduce fruit quality and overall tree vigor. This guide offers practical, sequential steps for removing aphids and preventing their return.
Identifying the Pest and Damage
Plum trees host several aphid species, appearing as small, soft-bodied, pear-shaped insects often less than 3 millimeters long. They can be pale yellowish-green, brownish-green, nearly black, or sometimes dusty whitish-green due to a waxy coating, like the mealy plum aphid. The most noticeable sign of feeding is the severe curling or distortion of young leaves and shoot tips, caused by chemicals injected into the plant tissue. Aphids cluster densely on the undersides of leaves, and their sticky honeydew is often followed by the development of black sooty mold.
Immediate Physical and Horticultural Controls
Direct, non-chemical intervention is the first step against localized aphid populations. A strong, targeted jet of water from a garden hose is an effective physical method for dislodging aphids from the foliage. Direct the spray at infested areas, particularly the undersides of leaves, aiming to knock pests off without damaging new growth or fruit. Apply water in the morning so the foliage dries before evening, minimizing the risk of fungal disease development. For small clusters, manually remove them by wiping them off or gently squishing them.
Horticultural Control
Horticultural control involves selective pruning to eliminate heavily infested parts of the tree, instantly removing a large portion of the aphid population. Prune off the tips of new shoots and any severely curled leaves, as these areas are typically the most afflicted and provide shelter for the pests. Ensure all pruned material is immediately bagged and removed from the area to prevent the aphids from migrating back to the tree.
Natural and Biological Treatment Options
After physical removal, targeted treatments using organic materials can suppress the remaining population. Insecticidal soaps are contact-based treatments that work by disrupting the aphid’s cell membranes, causing dehydration and death. Use a commercially formulated soap and thoroughly coat all surfaces of the infested foliage, ensuring coverage of the leaf undersides. Since these soaps have no residual effect once dry, repeat treatments are often required every four to seven days to manage successive generations.
Horticultural Oils and Biological Control
Horticultural oils, such as Neem oil, act as both a contact killer and a repellent. The active compound, azadirachtin, interferes with the aphid’s ability to feed and reproduce. Follow the product label carefully and avoid application during high temperatures or when the tree is stressed, which can cause leaf burn. Applying oil during the dormant season or delayed dormant period is highly effective, as it suffocates overwintering aphid eggs on the bark and twigs. Introducing or encouraging natural predators offers a sustainable, long-term approach to pest control. Lady beetles, green lacewings, and syrphid fly larvae are voracious aphid eaters that can be purchased for release or attracted to the garden.
Preventing Future Infestations
Long-term management of aphid populations involves cultural practices that maintain tree health and discourage the pests from establishing colonies. Monitoring the plum tree regularly, checking the new growth and leaf undersides early in the season, allows for intervention before populations become severe. Early detection is important because once leaves curl, the aphids inside are protected from topical treatments.
Cultural Practices and Ant Management
Proper fertilization is a factor in prevention, as excessive nitrogen promotes soft, vulnerable new growth that aphids prefer. Switching to a slow-release or lower-nitrogen fertilizer reduces the tree’s attractiveness to these pests. Maintaining overall tree vigor through adequate water and care supports the plant’s natural defenses. Managing the ant population is also important because ants actively “farm” aphids, protecting them from natural predators in exchange for honeydew. Applying a sticky barrier, such as tree grease or a specialized horticultural glue, around the trunk prevents ants from climbing the tree and interfering with biological controls. Excluding the ants allows the ecosystem to rebalance, enabling beneficial insects to control aphid numbers effectively.