How to control longan trees after they are infected with eriophyid mites?
Longan (scientific name: Dimocarpus longan Lour.), also known as guiyuan or yizhi, is a large evergreen tree with a tall trunk. The fruit is harvested when it ripens and turns yellowish-brown in July to August. It belongs to the Sapindaceae family; the fruit can be eaten fresh or processed into dried products, and the flesh, seed, peel and root can all be used medicinally. It is native to southern and southwestern China, and longan is cultivated in many countries and regions around the world, such as China, Thailand, Indonesia, Queensland in Australia, and Hawaii and Florida in the United States.
The longan eriophyid mite (Eriophyes dimocarpi) belongs to the class Arachnida, order Acari, family Eriophyidae. It is a newly emerging pest mite in longan production and a new mite species discovered only in recent years. It damages longan leaves and causes massive leaf drop in spring, especially in plants with weakened tree vigor, where defoliation is more severe, leading to reduced yield or even no fruit. Because its body is small and difficult to see with the naked eye, it is often mistaken for a disease symptom, or mistaken for chlorophyll discoloration caused by insufficient light, so targeted control measures are not taken in time. Therefore, it is necessary to provide some introduction to it.
1. Morphological characteristics 1. Adult mite: body tiny, difficult to distinguish with the naked eye, carrot-shaped, pale yellow to grayish-white in color. Head small, concealed beneath the thoracic dorsal plate; the thoracic dorsum has tortoise-shell-like patterned markings in 3 transverse rows, with 7 to 8 pieces per row. The ventral surface has 2 pairs of short, small legs. The posterior half of the abdomen has about 40 rings on the dorsal surface and about 60 rings on the ventral surface. There is 1 pair of setae on both sides of the third abdominal segment. 2. Nymph: body shape similar to the adult mite, body length 0.077 mm, milky white, translucent, head rounded and blunt, abdominal rings obvious. 3. Larva: body shape similar to the adult mite, but the cephalothorax is larger than the abdomen and slightly rounder, the body is more tapered, length 0.065 mm, white, translucent. Legs 2 pairs, abdominal rings obvious. When approaching molting, the legs retract inward, the body surface appears membranous, and the whole body becomes rounded and arched. 4. Egg: helmet-shaped, diameter 0.04 mm, thickness about 0.013 mm, white, translucent, glossy.
2. Damage symptoms The adult mites, nymphs and larvae of the longan eriophyid mite can all feed on and damage longan leaves, but they only damage mature leaf surface tissues, inserting their mouthparts into leaf surface cells to suck sap; damage on the underside of leaves has not been found. After cells are destroyed, the damaged area first becomes moist in appearance, then turns grayish-brown, and finally becomes purplish-brown. Photosynthesis in damaged leaves decreases or is even completely lost, and their lifespan is shortened. Because the leaf surface cuticle is not destroyed, the leaf surface does not lose its luster. Especially in winter, when damage symptoms appear in large numbers, it is often mistaken for seasonal physiological changes, or because the mites cannot be seen, it is thought to be caused by disease.
3. Occurrence pattern The longan eriophyid mite causes damage throughout the year, with multiple generations per year and overlapping generations. Cloudy days, poorly lit areas, or dry and cloudy weather in winter all favor its occurrence and damage. This mite has the habit of gathering to cause damage; it first damages shaded places, then spreads outward, living a semi-free life.
4. Control measures For densely planted orchards, be willing to cut down trees so that the orchard planting density is reasonable and ventilation and light penetration are good. Shaded inner canopy branches, drooping branches or excessively dense branches should be pruned promptly. Once damaged shoots are found, they should be pruned immediately and burned. For chemical control, spray affected plants with 20% dicofol emulsifiable concentrate diluted 750 times with water, or 15% pyridaben emulsifiable concentrate diluted 1500 times with water, or Chongmanba emulsifiable concentrate diluted 2000 times with water.