Damage Patterns and Control Methods of Spider Mites on Ginkgo Trees
Ginkgo biloba is a large deciduous tree, with a trunk diameter at breast height reaching up to 4 m. The fruit of the ginkgo tree is commonly known as white fruit (baiguo), hence the ginkgo is also called the white fruit tree. The ginkgo tree grows slowly and has an extremely long lifespan. Under natural conditions, it takes more than twenty years from planting to bearing ginkgo fruit, and only after forty years can it produce fruit in large quantities, so some people also call it the “grandfather-grandson tree”, meaning “the grandfather plants it and the grandson eats the fruit”. It is the longevity star among trees, with ornamental, economic, medicinal and other values.
Although the ginkgo tree can live for a thousand years, it often suffers from spider mite infestation.
I. Spider mite damage patterns:
The spider mites that damage ginkgo locally are mainly the hawthorn spider mite (Tetranychus viennensis). After overwintering, the spider mites emerge from dormancy in March and April to cause damage, and their early numbers are very small. Because some courtyard ginkgo trees have many natural enemies, which often prey on the spider mites completely, some courtyard ginkgo trees have no overwintering spider mites; the spider mites first damage other fruit trees, and then transfer from those fruit trees to ginkgo trees to cause damage. In June, spider mite damage can be seen on the leaves of courtyard ginkgo trees. July and August are the peak damage period, and the damage can last until September. From September to October, the spider mites overwinter in various developmental stages. After the leaves are damaged, they turn pale white, and in severe cases they become scorched and dry and fall off early. This affects the growth, fruiting and tree vigor of ginkgo.
II. Control methods
1. Manual control: On ginkgo trees with relatively light damage in summer, the spider mites overwinter later, generally overwintering in soil crevices and other places in the lower part of the trunk. In winter, use fresh soil to bury and cover the spider mites in the ground around the trunk to prevent them from emerging from the soil and climbing the trees in spring. In autumn and winter, clean up dead branches, fallen leaves and weeds under ginkgo trees in the courtyard, sweep them outside and burn them to reduce the overwintering population of spider mites.
2. Chemical control: For ginkgo trees with overwintering spider mites, after the spider mites emerge from dormancy in spring, spray 0.3°Be lime sulfur mixture 1-2 times to kill the spider mites. In July and August, when spider mite damage is very severe and natural enemies cannot effectively control the spider mite damage, spray a 3000-fold dilution of 20% clofentezine suspension. When there are many natural enemies of spider mites on the tree, and pests such as stinging caterpillars and bagworms are damaging the leaves on the tree, do not spray highly toxic pesticides that kill natural enemies. Instead, use biological pesticides or other low-toxicity pesticides that do not harm natural enemies to control other pests.
3. Biological control: There are many kinds of natural enemies that control spider mite damage locally, including the six-spotted thrips (Scolothrips takahashii), green lacewings, Stethorus punctillum, spiders, and others. Based on years of observation, the dominant natural enemy controlling spider mite damage locally is the six-spotted thrips.
In some years, spider mite damage on ginkgo is relatively severe while six-spotted thrips are scarce. In such cases, the method of augmenting six-spotted thrips can also be used: move the large numbers of six-spotted thrips occurring on crops such as kidney beans and eggplants in farmland onto the parts of ginkgo trees with more spider mites. Pick some leaves with a large number of six-spotted thrips and place them on the ginkgo trees, which can promptly control spider mite damage.
III. Precautions for spider mite control: When spider mites are damaging ginkgo, do not spray highly toxic pesticides that kill natural enemies on the ginkgo while six-spotted thrips are preying on the spider mites. In addition, only after spider mites have been damaging and reproducing on ginkgo for some time can natural enemies prey on them completely. Spider mite damage in June and July causes some damage to the leaves, but before serious damage occurs, do not rush to spray. Observe the activity of the spider mite natural enemies. When the ratio of the natural enemy six-spotted thrips to adult spider mites is 1:5, spraying is not necessary. If natural enemies are few and spider mite damage tends to worsen, chemical control can be used to promptly control the spider mite damage.