Characteristics of root maggots at the seedling stage of watermelon and suggestions for prevention and control
China's watermelon planting area and output ranked first in the world (Wang Shaoli and Zhang Youjun, 2017), according to the data of "China Rural Statistics", in 2018, China's watermelon planting area reached 1.5179 million hm2, and the total output was 61.5369 million t. In the production of watermelon, it is often used to cultivate strong seedlings by means of grafting seedlings and matrix cavity tray seedlings (Wang Juanjuan, 2017). However, the large amount of plant secondary biomass emitted by the concentrated seedlings of grafted wounds and burrows and their odors will attract some pests, such as root maggot pests, to lay eggs, which will have a great impact on the healthy seedlings and later production of watermelons. From January to the end of March 2020, root maggot pests were found in Sheyang County, Yancheng City, Jiangsu Province, Shucheng County, Lu'an City, Anhui Province, and Yizhang County, Chenzhou City, Hunan Province, which were difficult to control. To this end, the author puts forward prevention and control measures and suggestions from the aspects of agricultural control, physical control and chemical control, in order to provide key technical support for the healthy seedlings of watermelon and green pollution-free production.
1 The occurrence of root maggots at the seedling stage of watermelon is harmful
The main species of root maggots that occur at the seedling stage of watermelon are fly and mushroom mosquitoes, such as the grey ground species fly (Delia platura (Meigen)) and the Leek Late-Eyed Mushroom Mosquito (Bradysia odoriphaga). The developmental stages of this type of pest include adults, eggs, larvae and pupae, the adults are small flies or small mosquitoes, black or black-brown, winged and able to fly; the larvae are mainly pest states, collectively known as root maggots, with a body length of 5.0 to 9.0 mm, a black head, and a smooth translucent and footless body wall (color plate 1-a). Root maggots have a wide range of hosts, such as leeks, green onions, garlic, Chinese cabbage, carrots, lettuce (lettuce), watermelon, etc., and have strong reproductive ability. The developmental periods of species flies and mushrooms are different, with 2 to 4 generations per year and 32 to 62 days (days) per year in the breeding flies, and 3 to 6 generations per year in the developmental calendar and 23 to 37 days (days) in the developmental calendar. In addition, compared with the mushroom mosquito, the breeding fly has a longer lifespan, stronger flight ability, larger individuals, and is more harmful to watermelon plants.
Root maggots are insidious, usually clustering their larvae in the ground to feed on the roots of watermelon seedlings, or at graft wounds. After the seedlings are damaged by the graft wound, they are water-stained, the tissues soften to decay, which seriously affects the healing (color plate 1-b); after the seedling stem is damaged, the stems are hollow, softened, brown, and even decayed (color plate 2); after the roots are damaged, the roots are significantly reduced by the healthier plants (color plate 3), and the upper leaves of the plant are wilted, which is easily confused with blight. Therefore, it is difficult to identify the root maggots in the early stages, and once they are discovered, they have caused more serious damage to the melon seedlings.
2 Sources and pathways of maggot occurrence
The root maggots that harm watermelon seedlings come from a variety of ways: (1) when raising seedlings, the substrate is not sterilized and carries the eggs, larvae and pupae of root maggots; after seedlings, the eggs hatch under suitable conditions and begin to be harmful, resulting in the death of seedlings, and the death rate of dead seedlings is as high as 50% in severe cases. (2) When colonizing, the organic fertilizer or farm manure applied is not fully decomposed, and the smell emitted by it induces the seed flies to lay eggs, resulting in lack of seedlings and ridges. (3) The source of insects that stay in the soil for wintering and summering of former stubble crops. In addition, watermelon seeds carry insect eggs, or adjacent crops with leeks, garlic and other root maggots can also be cross-transmitted. In summary, the root maggots in the seedling matrix, organic fertilizer, soil and other media are the main source of pests in watermelon production, and they are also artificially controllable insect sources. Therefore, the root maggots should be scientifically and reasonably prevented and controlled from the "source".
3 Maggot control measures
Following the plant protection policy of "prevention first, comprehensive prevention and control", comprehensive prevention and control is based on "agricultural prevention and control, physical control and chemical control as a supplement".
3.1 Agricultural control Adjusting and improving the growing environment of watermelon and creating clean soil or media conditions that are not conducive to the development of root maggots are the keys to agricultural control. Soak the seeds in hot water at 55 °C before sowing, maintain the water temperature at 55 °C for 15 min (min), then naturally drop to about 25 °C and continue to soak the seeds for 6 to 8 h (hours), and wash off the eggs and germs on the surface of the seeds. When raising seedlings in the pit disc, the organic fertilizer formulated with the substrate must be fully decomposed. After sowing, a layer of grass and wood ash about 0.5 cm thick can be sprinkled on the surface of the pit disc to keep the surface of the burrow dry, which is conducive to reducing adult egg laying or preventing egg hatching. If the treatment is not timely or the treatment effect is not good, resulting in the occurrence of maggot damage, it is recommended to pull out the dead seedlings and take them out of the outdoors for timely destruction, and then treat the matrix in the hole plate with agents. Before the seedlings are transplanted, the countryside should be cleaned to keep the field clean; after the watermelon harvest, the plant residue should be cleaned up in time; the soil should be ploughed and dried when the field is rested, so as to reduce the number of root maggots in the stubble crop.
3.2 Physical control Before seedlings, in order to cut off the spread of root maggots from the "source", thorough disinfection of the seedling matrix after preparation is the basis. It is recommended to purchase a thoroughly disinfected substrate from a regular manufacturer. If you prepare a small amount of substrate yourself, you can carry out lamination disinfection in sunny weather, that is, the substrate is directly stacked on the outdoor cement floor, or a layer of insulated dropletless film is first laid on the soil floor, and then the matrix is stacked on the membrane to block the root maggot larvae carried by the matrix from entering the soil to pupate; and then cover the substrate with a light blue dripless film with a thickness of 0.10 to 0.12 mm, generally covering about 5 d (days), which can also be appropriately extended to fully kill the insect source that may be carried in the matrix. Before transplanting in the open field, choose a sunny weather, cover the field to be planted with a light blue dripless film of 0.10 to 0.12 mm thick, the edge of the film is 40 to 50 cm beyond the edge of the field, and pressed tightly with soil (Shi et al., 2018); it can also be watered with a concentration of 10 to 30 mg under the condition of lamination closure · L-1 ozone water (Hu Jingrong et al., 2018), generally covers about 3 d (days). If you are not in a hurry to transplant, it is recommended that the lamination time be extended as much as possible, which can not only kill the root maggots in the soil, but also kill other insect pests that are not resistant to high temperatures. Before the greenhouse is transplanted, the shed can be closed and watered with ozone water according to the above method, or the high temperature can be carried out in the summer. After transplanting in the open ground, mesh chamber and greenhouse, yellow armyworm boards can be hung at a height of 10 to 20 cm above the ground to monitor or trap adult maggots, and 20 to 30 sheets can be suspended every 667 m2. You can also use the chemotactic properties of root maggots to make a booby trap, mash the garlic cloves and leek false stems into a plastic basin, and then add 90% of the enemy worm crystals 1 000 times liquid 1 L to the pot, and place 5 to 10 pots every 667 m2 in the mesh chamber or greenhouse, and replace them once a week, which can lure and kill a large number of root maggot adults. It should be noted that it is not recommended to use booby traps to control root maggots in open watermelon fields, so as not to be counterproductive.
3.3 Chemical control Using scientific application methods, choosing a reasonable application period is the key to exerting the best efficacy. At present, the highly effective agents for the control of root maggots are thiamethoxamine, thiamethoxine, imidacloprid, furazine, high efficiency cypermethrin, octylthion, fly amine, carbonitrile, fludaride and so on. Before seedlings are bred, the shed should be thoroughly cleaned, and if necessary, it should be closed and fumigated with a cigarette agent to kill the original diseases and insects in the shed. If root maggots are found in the seedling stage burrow disc, or if adult root maggots are detected by yellow armyworm plates, 10% thiamethoxamine suspension 225 to 250 mL or 25% thiamethoxine water dispersible granules 180 to 240 g, or 70% imidacloprid wettable powder 29 to 43 g, or 50 g per 667 m2 L-1 fluorodinylurea emulsion 200 to 300 mL, or 70% octylthion emulsion 351 to 560 mL, or 4.5% high efficiency cypermethrin emulsion 35 to 50 mL, etc. diluted into 1 000 to 2 000 times the liquid, loaded into a large tank, the seedling tray is immersed in the sink for 3 to 5 s (s) to ensure that the matrix is soaked in the liquid; the diluted liquid can also be loaded into the spray bucket, unscrew the nozzle at the front of the spray rod, and spray one by one at the base of the seedling, so that the liquid penetrates the matrix and flows out from the bottom of the pit disc.
It is also possible to use 2% high-efficiency cypermethrin tobacco 225 to 270 g per 667 m2, or 15% dichlorvos 500 to 600 g, or 10% iproprovir tobacco 300 to 450 g, etc. for closed fumigation at intervals of 7 d (days), and if the early seedlings are more infested, it is recommended to fumigate every 7 d (days) until transplanting. After the seedlings are planted, if the root maggots are found in the shed, the number of adult worms on each sticky worm board reaches 7 to 10, and the fumigation can be closed according to the fumigation method at the time of seedlings. If maggots are found to be harmful in the open field, you can choose the above agents and doses to spray first, and then water a large amount of water, so that the water penetrates into the soil 4 to 5 cm deep. Or mix 1 kg of granules of the above drugs with 15 to 20 kg of fine soil or sand to make poisonous soil and sprinkle it on the root of watermelon seedlings.
4 Recommendations
The hidden pest characteristics of root maggot pests make it difficult to control the insect, and different prevention and control measures need to be comprehensively applied. First of all, attention should be paid to the selection and cleaning of seeds, ensuring that the seedling matrix and burrow plate have been disinfected, keeping the soil clean, and avoiding the development of potential insect eggs. Second, focus on operational hygiene, reduce the spread of pests through fertilizers and human activities. Finally, where conditions permit, insect nets can be used in combination to isolate the spread of alien sources.
Controlling the "source" of root maggots is the key to reducing their later damage. At present, the solar high temperature lamination method invented by the Institute of Vegetables and Flowers of the Chinese Academy of Agricultural Sciences is simple to operate, economical and practical, and if it is used scientifically and rationally, it can control the pest hazard from the "source". Without affecting the planting plan, the sun-based high-temperature lamination method can be used to kill the matrix, burrowing, fertilizer and root maggots carried in the soil, and other high-temperature-intolerant pests and insect pests can also be eliminated, while accelerating straw decomposition and machine decomposition in the field, promoting crop growth and increasing yield (Shi Caihua, 2017). If the early operation is improper, or the best prevention and control period is ignored, resulting in watermelon seedlings being harmed by root maggots, comprehensive prevention and control measures of "agricultural control, physical control and chemical control" can be adopted, and it is not appropriate to simply blindly use chemical agents.
