Citrus spinosus, commonly known as the arrowhead scale, and its scientific name, Unaspis yanonensis Kuwana, is a species belonging to the order Hemiptera and family Diaspididae. This pest is particularly damaging to a variety of plants, including citrus, toon, pomelo, longan, tea, orchids, and other fruit and ornamental crops.
The pests are typically found on the leaves, branches, and fruits of citrus plants. Both females and nymphs remain stationary, feeding on plant sap. Their presence leads to yellowing and premature dropping of leaves, branch dieback, and unripe or sour fruits, which significantly reduces both the quality and yield of the crop.
In terms of appearance, adult female insects have an arrowhead-shaped body with a broad anterior projection and a central longitudinal ridge. They measure between 2 to 4 millimeters in length, with a color ranging from yellow-brown to tan and a grayish-white margin. Their bodies are orange-yellow. Male scales are elongated and narrow, featuring three distinct ridges along their back. Males are smaller, measuring 1.3 to 1.6 mm in length, with a pinkish-white color and a waxy coating. The adult male has an orange-yellow body, about 0.6 mm long, with transparent front wings and a wingspan of approximately 1.7 mm. Nymphs have a yellowish shell and an orange-yellow body, and they are more developed than the adults in some aspects.
In South China, this pest completes three to four generations per year, with overlapping life cycles. Overwintering occurs mainly in the form of adult females. When the average monthly temperature reaches 19°C or higher in April, these overwintering females begin laying eggs. The first generation of nymphs peaks in early May, the second in mid-July, and the third in late September. Newly hatched nymphs are highly active and mobile for the first 1 to 2 hours before settling and feeding. By the next day, they start secreting wax, and they continue growing under their protective coverings. After three instars, they develop into adult females.
Natural enemies of Citrus spinosus include Japanese ladybugs, red-spotted ladybugs, and certain parasitic wasps. These beneficial insects play a crucial role in controlling the population of this pest.
For control, it's important to protect and encourage the use of natural predators. Additionally, targeted spraying during the early stages of each generation is essential. The first generation nymphs should be controlled within 20 days after the initial emergence. Subsequent sprays should be applied every 15 to 20 days. Recommended insecticides include 40% Lufenuron EC diluted at 1:1000, or Buprofezin (Undeletal) WP diluted at 1:1500 to 1:2000. For better results, these can be mixed with 50 times diluted oil emulsion. Always refer to the specific guidelines provided in your local agricultural handbook for accurate application methods.
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