Citrus spinosus, commonly known as the arrowhead scale, and the arrowhead shield (Unaspis yanonensis Kuwana), are members of the order Hemiptera and family Diaspididae.
Hosts and Damage: This pest affects citrus, toon, pomelo, longan, tea, orchids, and various fruit and ornamental plants. Both females and nymphs attach themselves to leaves, branches, and fruits, causing yellowing and premature leaf fall, branch dieback, and unripe, acidic fruits. Severe infestations significantly reduce both the quality and yield of crops.
Morphology: Female adults have an arrowhead shape, with a broad front spine, a central longitudinal ridge, and measure 2 to 4 mm in length. Their color ranges from yellow-brown to tan, with a grayish-white margin and an orange-yellow body. Male scales are elongated and narrow, featuring three longitudinal ridges on their back, measuring 1.3 to 1.6 mm. They appear pinkish-white and waxy. Adult males are orange-yellow, about 0.6 mm long, with two transparent wings and a wingspan of 1.7 mm. Nymphs have a yellowish shell and an orange-yellow body, with structures that are more developed than those of adults.
Life Cycle: In South China, the species completes three to four generations per year, with overlapping life stages. Overwintering occurs primarily in female adults. When the average monthly temperature reaches 19°C or higher in April, overwintered females begin laying eggs. The peak periods for the first, second, and third generations occur in early May, mid-July, and late September, respectively. Newly hatched nymphs are highly active and move around for about 1–2 hours before settling and feeding. By the next day, they start secreting wax, and continue growing under their protective coverings. After three developmental stages, they mature into adult females. Natural enemies include Japanese ladybugs, red dot ladybugs, and certain parasitic wasps.
Control Methods
1. Encourage the conservation and use of natural predators to maintain ecological balance.
2. Focus on spraying during the early nymphal stages of each generation, especially targeting the first generation. The best time to spray is approximately 20 days after the first emergence, followed by additional sprays every 15–20 days. For example, apply 40% Lufenuron EC diluted at 1:1000, or Buprofezin (Undeletal) WP at 1:1500–2000. Mixing with an oil emulsion at 1:50 can enhance effectiveness. Always refer to the specific guidelines for Citrus sinensis (see page 28 of this book) for detailed application instructions.
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