What Is the White Powder Under Some of the Leaves on My Mexican Faux Orange Tree?

Mexican faux, or mock, orange (Choisya ternata) smells wonderful all year, thanks to its orange-scented, deep green leaves and perfumed summer and spring blooms. This evergreen shrub grows in U.S. Department of Agriculture plant hardiness zones 7 through 10. Its troubles are several, and one of the rarest is a powdery, white bug nymph that feeds on the backs of its leaves.

The Citrophilus Mealybug

In the 1920s, the sap-draining citrophilus mealybug (Pseudococcus calceolariae) has been decimating thousands of citrus orchards. As a non-native, the pest had no natural enemies. On a detective mission to Australia, University of California biological management scientist Harold Compere found and imported two citrophilus-parasitizing wasp species. After being forced to do their work, the wasps nearly — but not entirely — eradicated the pests. While it still infests citrus trees (Citrus spp.) , citrophilus and its nymphs, or crawlers, also feed sometimes on Mexican orange. Citrus trees typically grow in USDA zones 9 through 11.

Nymphs vs. Adults

When white powder dusts the backs of its leaves, Mexican mock orange is hosting citrophilus nymphs, or crawlers. Wind, tools, clothing or animals spread the very small crawlers to the plant if no adults are present. Shortly after hatching, the crawlers start covering themselves with a protective layer of powdery, white wax. Nevertheless, they are too small to be distinguishable with no 10X magnifying lens. At about 1/16 inch long, the adult females are easier to see their heavenly, white egg sacs, they look from a distance like pieces of cotton wool.

Possible Damage

Citrophilus mealybugs weaken a Mexican mock orange by draining sap from its leaves and stalks. In addition they drench the plants at honeydew, their gooey, clear waste. Windborne sooty mould spores feeding on the honeydew cover the leaves with layers of oily, black fungus. It does not tighten the leaf tissue, but a thick coating can interfere with photosynthesis and tighten the plant much longer. The crawlers reach adulthood in two to three months, and every single adult female lays up to 500 eggs. Without rapid treatment, a couple crawlers might become a serious threat to the tree’s health.

Cultural Crawler Control

The fastest way to rid your Mexican mock orange of citrophilus crawlers is to direct a forceful blast of water from the hose in the plant until all of its surfaces drip. If infested citrus trees are nearby, the end might be making regular crawler deposits on your plant. Monitor the plants and spray as required to dislodge new infestations.

Chemical Crawler Control

The parasitic wasps (Coccophagus gurneyi, Tetracnemus pretiosus) released in the 1920s continue to maintain citrophilus in nonthreatening levels. If the crawlers need chemical treatment, ready-to-use insecticidal soap suffocates them on contact, but does not harm the wasps after it dries. When no sunlight is hitting the shrub, spray until the soap drips from either side of the leaves. Repeat every a couple of weeks, or even in the brand’s recommended period, until the infestation stops. Wear long pants, a long-sleeved shirt, hat and protective eyewear, and follow the label instructions when spraying.

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