Occurrence and Control Measures of Oulema tristis Herbst in Jincheng City

Xueping SHANGGUAN

Jincheng Bureau of Agriculture and Rural Affairs, Jincheng 048026, China

Abstract In recent years, with the increase of millet market demand and the promotion and application of millet free (less) thinning technology in Jincheng City, the planting area of millet is stabilized at about 10 000 hm2. However, in the process of planting, millet farmers have not paid enough attention to the control of Oulema tristis Herbst, and the damage caused by the pest is aggravating year by year, causing certain loss to millet farmers. Based on the investigation and analysis of the occurrence of O. tristis, the prevention and control suggestions are put forward, and good results have been obtained.

Key words Millet; Oulema tristis Herbst; Control measures

1 Introduction

Jincheng City is located in the southeast of Shanxi Province, and there are three mountains within the territory, including Taihang Mountain, Zhongtiao Mountain, Taiyue Mountain. Because of large mountains and deep valley, the microclimate is very obvious. 90% of The cultivated lands in Jincheng City are mountainous and hilly lands, most of which are dry lands. The layers of terraced fields are thin and uneven. Most of the places suitable for planting millet are in mountainous and hilly areas, with an altitude of 800-1 000 m, four distinct seasons, rain and heat over the same season, large temperature difference between day and night, and it is far away from the town without pollution. The millet produced is natural pollution-free product. In recent years, due to the increasing demand for millet, and millet free (less) thinning technology has been vigorously promoted in the city, and millet planting area has been stabilized at 10 000 hm2in recent years. However, in the process of planting, millet farmers generally pay more attention to the prevention and control of pests and diseases with relatively heavy damage such as underground pests, borer and millet downy mildew, but do not pay enough attention to the prevention and control ofOulematristisHerbst with relatively light damage, resulting in aggravating damage ofO.tristisyear by year. The yield reduction of plots severely damaged is 10%-15%, causing certain loss to millet farmers. Therefore, the technical personnel of municipal bureau of agriculture and rural affairs have seriously investigated and analyzed the occurrence ofO.tristisin the past two years. The prevention and control suggestions are put forward, and good results have been obtained.

2 Life habit and occurrence regularity of Oulema tristis in Jincheng city

2.1 Life habitO.tristisbelongs to Chrysomelidae, Coleoptera. In spring, after awakening of insects, overwintering adults begin to revive, which first feed on mesophyll of rejuvenated weeds. After the seedling emergence of millet,O.tristiswill migrate to millet field to damage millet seedlings. The adults have the characteristics of feign death and phototaxis, with certain flight ability.O.tristisgenerally does not feed during the daytime, but only makes short distance migration. Most of adults are perched on the back side of the leaves of millet seedlings, crawl to the upper leaves of the plant in the evening to feed, or migrate to the leaves of neighboring plants for courtship, copulation and oviposition. The newly hatched larvae crawl slowly, and most of them infiltrate into the heart leaves of millet seedlings or the leaf sheaths close to the heart leaf to harm the growth of crops. They feed on the mesophyll, remaining veins and epidermis, and forming white longitudinal stripes on leaves. In addition,O.tristislarvae have cannibalism.

2.2 Occurrence regularityIn the 6 counties (cities, districts) under the jurisdiction of Jincheng City,O.tristisonly occurs one generation a year. The overwintering adults feed and copulate in the leaves of millet seedlings, and lay their eggs on the back side of the leaves near the midrib, most of which are laid on the third leaf on the basal part of stem. The spawning frequency of a female adult in her lifetime varies with the climate, usually 5-11 times. The number of eggs laid by a female adult each time is also different, ranging from the minimum of 3 eggs to the maximum of 20 eggs. The eggs are usually arranged in the shape of straight line, and the egg period is about 7 d. The egg is light yellow at the beginning and gradually becomes black before hatching. The egg is resistant to drought and rain erosion, and the hatching rate is high. The larval stage ofO.tristisis approximately 20 d, which can be divided into four instars, and each instar is not the same. There are at least 3-5 larvae or more than a dozen in heart leaves of the same millet seedling. The first instar larva experiences 6 d or so, which mainly migrate from the back side of leaves to the heart leaf of millet or the leaf sheath close to heart leaf, with small appetite and light damage. The second instar larva lasts about 6 d, during which the appetite begins to increase. The third instar and the forth instar larvae last 5 d, with large appetite and high degree of damage; the width and length of leaf mesophyll increases significantly, and the chlorotic white patches are not only plentiful, but also large. Due to frequent mating of adults, the spawning period is long, resulting in irregular generations. Adults, eggs and larvae can be seen in the fields in mid and late June. With the growth of new heart leaves, the larvae continue to migrate and hide in the heart leaves of millet seedlings until the 4thinstar larvae become mature. The mature larvae fall to the ground, and drill into the topsoil of loose and moist soil for cocooning and pupation. The pupal stage is about 20 d, and contemporary adults successively appear since mid-July. Adults migrate to weeds, stubble and soil crevices in mid-September as the weather becomes cold.

3 Causes of the occurrence of Oulema tristis in Jincheng city

3.1 Winter warmingCold winter or much winter rain and snow can freeze part of the overwintering adults to death, and the initial quantity of overwintering adults is small, so the damage in the next year will be lighter. On the contrary, less rain and snow in winter, drought or warm winter will lead to large initial quantity of overwintering adults, so the damage in the next year will be heavy.

3.2 Early sowing timeMillet in Jincheng is mostly planted in the mountains and hills, with uneven soil, so the planting habits of millet farmers are not exactly the same. In order to ensure the emergence of seedlings, millet farmers begin to sow millet from April 20, and farmers in Jialing Township of Yangcheng County have the habit of planting winter millet, resulting in different occurrence time and harmful degree ofO.tristis. The damage degree of sowing before May 5 is more severe; the damage degree of sowing between May 5 and May 15 is light, and those sowing between May 15 and May 23 are basically not affected.

3.3 Irrational crop rotationThere is an agricultural proverb that no rotation of millet will result in yield reduction. Although the local millet farmers rotate crops every year, the phenomenon of unreasonable crop rotation sometimes appears. The first reason is that millet is mostly rotated with corn due to the influence of millet farmers’ planting habit, grain price and yield, andO.tristisadults harm both the young leaves of millet and the young leaves of corn. The second is that the planting area with village as a unit lacks planning, and millet is often planted across adjacent plots. Such crop rotation instead provides a continuous and environmentally suitable living space forO.tristis.

3.4 Millet farmers do not pay enough attention to the control ofO.tristisThe initial symptoms ofO.tristisadults are not obvious, and there is only a very short crack on the leaves, which is difficult to attract people’s attention. When millet enter into the early and middle jointing stage, a large number of white coke leaves appear, but the best control period has been missed. Pesticide control at this time often receives poor results, and millet farmers are frustrated, helpless, some even give up prevention and control. By late June, with the coming of the rainy season, the increase of precipitation has effectively controlled the harm ofO.tristislarvae, and the growth of plants has returned to the normal growth trend, but the harm in the early and middle stages will still have negative impact on the overall yield. The ears of affected plants are smaller than that of unaffected plants, and the yield level decreases. However, the majority of millet farmers are mainly self-sufficient, so it is difficult to cause millet farmers to pay attention to the prevention and control ofO.tristis.

3.5 Neglect to clean up weeds, stubbles and residual plants

Due to the limitation of planting scale and labor amount, it is difficult for farmers to estimate the weeds at the ridge of fields and crop residues in the fields, resulting in severe insect residues. The field investigation suggest thatO.tristisadults survive the winter in millet stubble, ridge slope cracks, and weed roots, litters, crop residues. The population density of overwintering adults is larger when it is closer to barren hills and slopes or there are more weeds at the edge of fields.

4 Control measures

4.1 Agricultural measures

4.1.1Rational crop rotation. Planning should be done well with village as a unit, and millet is planted contiguously in concentrated area for rational rotation, so as to avoid random planting of one family/household as far as possible. It is best to choose potatoes, sweet potatoes, beans and other crops as the first crop.

4.1.2Clean up overwintering hosts. The weeds at the ridges of the filed, as well as residual plants and stubble in the field, should be timely cleaned up, and concentratedly burned and deeply buried, to destroy the overwintering sites of adults and reduce the density of overwintering population. Especially millet fields should be cleaned up after millet harvest as early as possible, and millet remnants and stubbles must be picked up completely, so as to eliminate living conditions and environment forO.tristis.

4.1.3Land preparation by deep ploughing. After the autumn crops are harvested, the stubble is destroyed in time, and weeds are removed in the field; 2 000-3 000 kg of decomposed farm manure is applied, and deeply plowed for about 25 cm. The land is harrowed after ploughing for surviving the winter. The soil can also be deeply plowed for over 35 cm, and timely conduct rotary tillage. The field surface is leveled to make the ground leveling and soil soft, without any soil seams, so as to destroy soil crevices where overwintering adults inhabit.

4.1.4Appropriate late sowing. The suitable sowing time of millet in Jincheng City is generally from the beginning of summer to grain full (May 5-23), and appropriate late sowing is advocated. In order to make the millet farmers accustomed to early sowing can be voluntarily sown in accordance with this period, three measures are adopted. The first is to hold a training meeting to popularize science and technology, and inform millet farmers of the disadvantages of early sowing and the advantages of appropriate late sowing. The second is on-site demonstration. In concentrated planting areas of millet, sowing date demonstration sites are established, and on-site meetings are held in different growth periods,

so that millet farmers can see and experience with their own eyes. The third is the issuance of publicity materials. Activities such as "volunteer activities for the country people", field technical guidance, going to market and temple fairs are utilized to disseminate information to fields and villages and carry out secondary popularization of pest control knowledge.

4.2 Physical measures

4.2.1Promote the application of solar insecticidal lamps. Using the phototaxis characteristic ofO.tristisadults, the insecticidal lamps are installed in the planning area of millet field to trap and kill adults, which can effectively controlO.tristisand other pests.

4.2.2Plant trapping fields. With cooperative or village as the unit, a small area of trap fields is planted in advance in millet patches or large-scale planting areas, and the sowing time is around April 10. The overwintering adults are trapped in early sowing millet filed for concentrate elimination, which can effectively reduce the harm ofO.trististo the field.

4.2.3Concentrated trapping. By taking the advantage of feign death of adults, the adults were trapped concentratedly during peak period through field investigation. In late May when the rice seedlings grow to 4 leaves, 0.1 mm plastic shed film is cut into 50 cm wide and 200 cm long. Two people are cooperatively spread the film between ridges, and gently shake the left and right sides of plants, for artificial concentrated trapping of adults.

4.2.4Killing larvae by hand. When there are small dry white spots on millet leaves, it indicates that the larvae have begun to harm the plant. The heart leaf can be pinched from top to bottom by hand, to kill a large number of larvae.

4.3 Chemical controlThe 3-leaf period of millet is the peak oviposition period of overwintering adults, and it is also the best period for chemical control, so it is necessary to investigate the amount of eggs in the field. Chemical agents are sprayed when there are more than 3 eggs per plant. In a sunny evening, pyrethroid pesticide with high efficiency and low toxicity are diluted with water for spraying. The liquids should be spread on the back of the leaves. When millet seedlings grow to 8 leaves, it is the critical period for larvae to infiltrate into the heart of millet and harm crops. It is necessary to investigate the damage degree of leaves in the field and check the number of larvae in the heart of millet seedlings. When there are 2 or 3 larvae in a single plant, chemical prevention and control should be carried out. Prevention and treatment of toxic soil by irrigation is the preferred control method. 1.0-1.5 kg of 5% Chlorpyrifos· phoxim DL or 10% phorate·phoxim DL can be mixed with 25 kg of dry fine soil, and then broadcasted in heart leaves of millet seedlings along ridges to kill the infiltrating larvae. Spraying prevention and control can also be adopted[1]. Pyrethroid pesticides with high efficiency and low toxicity are selected to spray in sunny and windless days, and the heart leaves and leaf sheaths close to the heart leaves of millet should be focused during spraying. According to the field investigation, the damage rate ofO.tristisin Jincheng City in 2021 has been controlled less than 1%, and the damage degree is significantly reduced, with outstanding prevention and control effect. Under the joint efforts of the general millet farmers and agricultural technology extension personnel, the achievement of millet harvest and millet farmer income increase is remarkable.