Causes of Tea Geometrid Outbreaks and Pollution-Free Control Methods

Tea geometrid, also known as the arching worm, measuring worm, and hanging-silk worm, belongs to the order Lepidoptera, family Geometridae. In Liandu District, this pest is mainly distributed in tea areas such as Bihu and Lixin, and is a voracious pest. If control is not timely, it often causes severe reductions in summer and autumn tea yields, and leads to decline in tree vigor, seriously affecting tea production. In previous years it occurred every year, but the damage was not great. In August 2007, this pest broke out over a large area in the Huanglingshang tea garden of Lixin Township, not only severely affecting the tea yield of that season, but also causing decline in tree vigor, poor cold resistance, and susceptibility to freeze damage, affecting tea production the following year.

1 Occurrence

On August 19, 2007, a small number of larvae were found in the Huanglingshang tea garden of Lixin Township. Seven days later, a relatively large area of the tea garden was damaged, with all green leaves and tender branches of the tea plants eaten clean. In most tea plants in the affected garden, only the main veins of the leaves remained, resembling fire damage, and there was no tea to pick.

2 Habits and Damage Characteristics

2.1 Life Habits

Most tea geometrids overwinter as pupae in the surface soil around the roots of tea plants, while a few overwinter as larvae in the tea bushes. Most overwintering pupae are in a diapause state, with strong stress resistance and low mortality; their eclosion rate after overwintering is greatly affected by soil moisture.

In Zhejiang, there are 6 to 7 generations per year. The 1st and 2nd generations show an obvious "pest outbreak center" phenomenon. After the 3rd generation, generations overlap. The 4th generation has a large population and is the most damaging generation of the year. If environmental conditions in early autumn are suitable (temperature, food supply, and control not timely), the 6th generation of tea geometrid has a large population, causing severe autumn tea losses; if the temperature in early winter is above 20°C, a 7th generation may occur. After tea geometrid larvae hatch, they form obvious outbreak centers on the tea canopy. The larvae spin silk and hang down, spreading with the wind to cause damage. The 1st to 2nd instar larvae mostly feed on the leaf surface, and the 4th or 5th instar enters the voracious feeding stage.

2.2 Damage Characteristics

Wherever tea plants grow well, the tea garden soil is fertile, the soil layer is deep, the structure is loose, and water retention is good, the pest population is generally large, the density is high, and the damage is more severe; tea gardens sheltered from wind and facing the sun occur earlier and suffer more severe damage; flat and sloping tea gardens suffer more severe occurrence than mountain tea gardens; hilly tea gardens suffer more severe occurrence than high-mountain tea gardens.

3 Causes of the Outbreak

3.1 Climatic Factors

Due to the warm winter weather in 2006, the overwintering population of tea geometrid was relatively large, with relatively high maturity and a relatively high overwintering survival rate. This provided a sufficient population quantity for the outbreak of tea geometrid.

3.2 Food Factors

Due to the increase in newly developed tea gardens, improvements in tea garden fertilization and management, and changes in plucking systems, summer and autumn tea was left to grow too much; this provided abundant food for the outbreak of tea geometrid.

3.3 Untimely Control

Because in past years the occurrence was uneven and small in scale, it did not constitute a disaster, and therefore did not attract attention in production. At the same time, due to a lack of understanding of the occurrence characteristics of tea geometrid and the appropriate control period, attention was only drawn when tea geometrid entered the late-instar voracious feeding stage. By the time control was carried out, it had already entered the late-instar voracious feeding stage, causing heavy losses and increasing the difficulty of control.

4 Pollution-Free Control

4.1 Strengthen Forecasting and Prediction

In view of the occurrence characteristics of tea geometrid, pest monitoring must be strengthened, the dynamics of pest occurrence must be grasped in a timely manner, the transmission of pest information must be done well, and pest trend forecasts must be issued accurately, so as to provide a scientific basis for pest disaster control and make it convenient for tea farmers to take appropriate control measures in a timely manner according to the situation.

4.2 Timely Plucking and Deep Tillage

Tea geometrid larvae mainly feed on the tender leaves of tea plants. Timely plucking of fresh leaves can carry away some larvae and reduce damage. In combination with deep digging and fertilization in autumn and winter, the pupae of tea geometrid in the surface soil around the roots of tea plants can be buried deep in the soil, preventing them from emerging and eclosing, thereby reducing damage.

4.3 Trapping and Killing

Adult tea geometrids are phototactic. Taking advantage of the phototaxis of adults, frequency-vibration insect-killing lamps can be used to trap and kill adults during the moth emergence period, thereby reducing egg production. The mating disruption method can also be used to interfere with mating, that is, releasing a large amount of sex pheromone in the tea garden to disrupt the normal information connection between male and female adults, causing males to lose the directional ability to find females and be unable to mate, thereby reducing the incidence of larvae.

4.4 Manual Capture and Killing

According to the habit of larvae spinning silk and hanging down after being disturbed, they can be knocked down in the evening or before dawn and caught to be eliminated.

4.5 Spraying Virus

The nuclear polyhedrosis virus of tea geometrid has a strong infection rate against tea geometrid larvae. Throughout the year, the pathogenicity rate is highest in the 1st, 5th, and 6th generations. The dosage per mu is 15 billion to 70 billion polyhedra, and the application period should be controlled during the 1st and 2nd instar larval stages.

4.6 Protect and Utilize Natural Enemies

The natural enemies of tea geometrid include parasitic flies, parasitic wasps, spiders, lacewings, and ants. In tea gardens, the number of pesticide applications should be minimized and the amount of pesticide used should be reduced to protect natural enemies and give full play to their control role.

4.7 Chemical Control

The 1st and 2nd generations of this pest occur relatively uniformly, while the 3rd generation shows overlapping generations. In production, eliminating tea geometrid before the 3rd generation plays an important role in controlling the damage throughout the year. Chemical control should strictly follow the control thresholds. For mature tea gardens in production, the control threshold is 4,500 larvae per mu, or 10 larvae per meter of tea row; when this threshold is reached, the tea garden needs pesticide spraying for control. The appropriate application period should be controlled during the larval stage before the 3rd instar. The key generation for full-scale application is the 4th generation, followed by the 3rd and 5th generations, while the 1st and 2nd generations are recommended for spot treatment. The application method should preferably be low-volume sweeping spray on the canopy. For pesticides, 2.5% cyhalothrin can be selected, with 20 to 25 mL per mu and a pre-harvest interval of 5 days; 2.5% deltamethrin, with 20 to 25 mL per mu and a pre-harvest interval of 5 days.