Plum trees are prized for their sweet fruit and ornamental spring blossoms. While generally hardy, these trees are highly susceptible to insect pests, and infestations can significantly reduce fruit yield and compromise the long-term health of the wood. Successfully growing plums requires understanding the specific insects that attack the tree and implementing timely management strategies. Recognizing the physical characteristics of these pests is the first step in protecting your harvest.
Identifying Common Plum Pests
The most destructive insects attacking plum trees fall into three main categories: weevils, sap-suckers, and internal borers. The Plum Curculio (Conotrachelus nenuphar) is a small snout beetle, measuring approximately 5 to 6 millimeters long, with a mottled brownish-black body and distinctive humps on its wing covers. Its larvae are legless, grayish-white grubs with brown heads, often found tunneling within the fruit.
Aphids are small, soft-bodied, sap-sucking insects, typically ranging from 1 to 7 millimeters in length, and are found clustered on new growth. Species like the Mealy Plum Aphid are dusky green and covered in a powdery, white wax, while the Leaf-Curling Plum Aphid is shiny green and causes distortion of the foliage. Borers, such as the Peachtree Borer (Synanthedon exitiosa), are the larval stage of clear-winged moths that mimic wasps. The resulting larvae are cream-colored, brown-headed caterpillars up to 1.5 inches long that bore into the tree’s wood.
Recognizing Signs of Damage
Visual evidence of a pest problem often appears before the insect itself is seen, offering an opportunity for early diagnosis. Plum Curculio damage is characterized by a unique crescent-shaped scar on the surface of the developing fruit, which is the mark left by the female after laying an egg. These wounds can cause the fruit to become misshapen as it grows or lead to premature fruit drop, which is the tree’s mechanism for shedding infested fruit.
Aphid feeding causes leaves to tightly curl and become distorted, especially on terminal shoots, as the insects shelter and feed inside the protective folds. These sap-suckers excrete a sugary waste product called honeydew, which coats the leaves and fruit, leading to the secondary growth of a black, powdery fungus known as sooty mold. Evidence of Borers is identifiable by the presence of a thick, amber-colored sap, known as gummosis, oozing from the trunk or branches. When a borer is the cause, this resinous gum is typically mixed with reddish-brown, sawdust-like insect excrement called frass, usually concentrated near the base of the tree.
Seasonal Timing of Pest Attacks
Understanding the seasonal cycle of plum pests is fundamental to effective intervention. Plum Curculio adults emerge from overwintering sites in the spring when temperatures consistently reach approximately 60°F or exceed 75°F for several days. Egg-laying begins soon after the petals drop and the small fruit has set, often coinciding with the shuck-split stage of fruit development. This activity continues for several weeks in the early growing season.
Aphid management is most effective when timed with the hatching of overwintering eggs, which occurs at the earliest stages of tree development, such as bud break. Peachtree Borers operate on a different schedule, with adults flying and laying eggs on the lower trunk and root crown during the warmer summer months, typically from late June through September. The resulting larvae then feed internally throughout the summer and survive the winter within the tree before resuming feeding the following spring.
Effective Control and Prevention Methods
An integrated pest management approach combines cultural practices, physical controls, and targeted treatments to minimize pest populations. Sanitation is a foundational practice, involving consistently removing and destroying all fallen fruit during the early summer. This fruit contains developing Plum Curculio larvae that would otherwise mature and return as adults the following season. Reducing overwintering habitats, such as brush piles, leaf litter, and wild hosts near the plum tree, helps limit the number of emerging adult pests in the spring.
Physical controls offer a non-chemical solution for Plum Curculio. In the cool morning hours, before the beetles become active, a padded mallet can be used to jar the tree trunk, causing the weevils to fall onto a sheet placed below for collection and destruction. For borers, the practice of “worming” involves carefully inserting a flexible wire into the oozing gum mass at the trunk base to pierce and destroy the feeding larvae. For young trees susceptible to borers, wrapping the lower trunk with a physical barrier or cone before adult egg-laying begins in summer can prevent larval entry.
Targeted sprays are most effective when applied precisely based on the pest’s life stage and the tree’s development. Aphids can be controlled with a dormant oil spray applied in late winter before bud swell, which works by suffocating the overwintering eggs on the bark. If aphids are seen during the growing season, a strong jet of water or an application of insecticidal soap or neem oil can reduce populations. The spray must thoroughly coat the insects before they curl the leaves and gain protection. For Plum Curculio, applications of kaolin clay, which creates a fine, white, physical barrier on the fruit, can be started at shuck split to deter egg-laying. Borer control relies on targeted trunk sprays of permethrin or similar insecticides applied to the lower trunk and base in late May-early June and again in August or September to kill newly hatched larvae before they bore into the wood.