Olive trees are popular additions to landscapes, from backyard gardens to large commercial groves, prized for their silvery foliage and valuable fruit. Managing common insect pests that feed on the tree’s leaves, branches, and olives is necessary. Successful control begins with accurately identifying the specific problem, as each pest requires a distinct management approach.
Recognizing the Signs of Infestation
One frequent threat is the Olive Scale, or Black Scale (Saissetia oleae). It appears as small, sedentary, dark brown to black dome-shaped bumps, typically found on stems and the undersides of leaves. These insects feed on the tree’s sap and excrete honeydew, a sugary waste product. Sooty mold, a black fungus, thrives on this sticky honeydew, coating leaves and fruit and reducing the tree’s ability to photosynthesize.
The Olive Fruit Fly (Bactrocera oleae) causes damage primarily to the fruit. Female flies deposit eggs just under the skin of the olive, leaving a characteristic triangular puncture on the ripening fruit. The developing larvae tunnel and feed inside the olive pulp. This feeding causes premature fruit drop and allows secondary bacteria and fungi to enter, leading to rot.
The Olive Moth (Prays oleae) damages leaves, flowers, and fruit across its three generations. Larvae may bore tunnels, creating mines on the leaves, or spin silk threads to web together flower clusters, feeding on the blossoms. Later generations bore into small fruit, causing premature fruit drop, which is a visual indicator of this pest’s activity. Inspection should routinely focus on all parts of the tree to catch these problems early.
Cultural and Mechanical Removal Strategies
Before resorting to chemical treatments, physical and cultural methods can reduce pest pressure and promote tree resilience. Targeted pruning is an effective strategy, removing heavily infested branches that harbor scale insects or moth larvae. Thinning the canopy improves air circulation and light penetration, creating an environment less favorable for scale insects and sooty mold development.
Sanitation involves consistently clearing fallen olives and leaf debris from the base of the tree and the surrounding area. This practice is important for controlling the Olive Fruit Fly, as larvae and pupae drop to the soil beneath the canopy to complete their life cycle. For soft-bodied pests like scale crawlers or aphids, a strong, direct jet of water can physically dislodge them from the leaves and branches. This non-chemical, mechanical action can be repeated as needed, especially against newly hatched scale crawlers before they develop their protective outer shell.
Effective Treatment Applications
Applying horticultural oils is a widely used treatment for managing scale insects and their overwintering eggs. These oils function as a physical control, coating and suffocating the pests by blocking the spiracles (breathing pores). Dormant oil, a higher concentration, is typically applied in late winter or early spring before bud break, when the tree is leafless and insects are less active.
During the growing season, a lighter concentration, referred to as summer oil, targets vulnerable, newly hatched scale crawlers. Application should be avoided when temperatures exceed 90°F or when the tree is under water stress, as this can lead to phytotoxicity (plant damage). Another option for soft-bodied insects is insecticidal soap, formulated with potassium salts of fatty acids that disrupt the insects’ cell membranes upon direct contact. Like oils, insecticidal soaps have no residual effect, meaning the spray must thoroughly cover the pest.
For Olive Fruit Fly control, targeted trapping and bait sprays minimize the use of broad-spectrum insecticides. Pheromone traps and yellow sticky traps monitor adult fly populations and can be deployed for mass trapping. Lure-and-kill bait sprays, such as those containing spinosad, are applied as small, concentrated spots on the foliage. These sprays attract adult flies to a toxic substance, limiting the treatment area to a fraction of the tree.
Biological Control for Moths
For Olive Moth larvae, the biological insecticide Bacillus thuringiensis (Bt) is an option. This soil-dwelling bacterium specifically targets and kills moth and butterfly larvae when ingested.
Seasonal Timing and Preventative Care
Proactive management requires monitoring pest activity to ensure treatments are timed for maximum efficacy against the pest’s most susceptible life stage. For black scale, applying horticultural oil must coincide with the emergence of the mobile, unprotected crawler stage, which often occurs in late spring or early summer. Applying oil at the wrong time, such as against fully armored adult scales, yields poor results.
Dormant oil applications target overwintering scale eggs and adults. They are best performed in late winter or early spring when temperatures are above 40°F but before the tree breaks dormancy. Beyond direct pest control, maintaining the overall health of the olive tree is a preventative measure. Ensuring proper irrigation and soil health supports a strong, vigorous tree that is naturally more resistant to pest damage and better able to recover.