Damage Caused by the Rice Stem Maggot to Hybrid Rice Seed Production and Its Control Methods

The rice stem maggot (Chlorops oryzae Matsumura) is mainly distributed in China in Heilongjiang, Zhejiang, Jiangxi, Hunan, Hubei, Guangdong, Guangxi, Yunnan, Guizhou, Sichuan, Chongqing and other provinces and areas. In recent years it has become one of the main insect pests in rice production in the hilly and mountainous areas of Chongqing, and the losses it causes have exceeded those of some important pests such as stem borers. In hybrid rice seed production, the occurrence of the rice stem maggot seriously affects the development process of rice, thus leading to poor flowering-date synchronization between the male and female parents and affecting seed production yield. Based on experimental observation and records, the author has systematically described the biological characteristics of the rice stem maggot and its occurrence, damage and control techniques in hybrid rice seed production, for reference.

1 Morphological characteristics and main life habits of the rice stem maggot

1.1 Morphological characteristics The rice stem maggot belongs to the family Chloropidae of the order Diptera. The adult is 2-3 mm long, with a wingspan of 5-6 mm, and the body is bright yellow. There is 1 diamond-shaped black spot on the top of the head; the compound eyes are large and dark brown; the back of the thorax has 3 dark brown longitudinal stripes, and there is 1 black transverse band at the junction of each abdominal segment on the dorsal side; 1 small black dot is borne on each side of the dorsal surface of the first segment, and the ventral surface of the body is light yellow; the wings are transparent with brown veins; the legs are yellowish brown with the ends of the leg segments dark black; the egg is oval and white, 0.7-1 mm long; the larva is maggot-shaped, and when mature the body is 5-6 mm long, slightly spindle-shaped, with 11 segments, milky white or yellow, with the mouth hooks light black, and the tail end divided into 2 forks; the pupa is 6 mm long, yellowish brown, with black spots, and the tail end is also divided into 2 forks.

1.2 Life habits The rice stem maggot mainly has 2 generations occurring and causing damage per year in Chongqing, and the occurrence of each generation is extremely irregular. It overwinters as larvae on weeds such as Alopecurus aequalis, Leersia sayanuka, Poa annua and Echinochloa crus-galli. For the overwintering larvae, late March to late April is the peak pupation period, and late April to early May is the first egg-laying period. Adults mostly lay eggs on the underside of leaves, and the eggs are laid singly; the newly hatched larvae move downward along the underside of leaves with the help of dew and mainly damage the heart leaves of rice seed-production seedlings. The second egg-laying period is from mid-to-late June to late July, and after hatching they mainly damage young panicles at the young panicle differentiation stage. Each generation goes through a 1-month larval stage. After maturing, most crawl to the inner side of the leaf sheath of the plant to pupate, generally 1-2 pupae per sheath, and then emerge as adults, which migrate to overwintering host plants to lay eggs, mostly overwintering as larvae. It generally occurs above an altitude of 300 m, and the damage becomes more severe as altitude increases, with the occurrence time slightly delayed.

2 Occurrence and damage of the rice stem maggot in hybrid rice seed production

2.1 Occurrence of the rice stem maggot in hybrid rice seed production It occurs more in hybrid rice seed-production fields than in hybrid mid-season rice fields. In the process of hybrid rice seed production, in order to make the male parent grow well and tiller more, a large amount of nitrogen fertilizer is applied, so the growth is relatively tender and green, and at the same time the population density of the female parent is also relatively high, so that resistance is weakened, making it more susceptible to damage by the rice stem maggot; while hybrid mid-season rice is generally stronger in resistance than conventional varieties (including the male and female parents), and at the same time the transplanting density is relatively sparse, which is not conducive to the growth of diseases and insect pests. For example, in 2005 in a seed-production field in Shituo Town, Fuling District, Chongqing at an altitude of 300 m, a survey on May 2 found that the damaged plant rate of male parent seedlings without pesticide control reached 35%-40%, while the damaged plant rate of adjacent hybrid mid-season rice seedlings under the same management was 10%-20%; from 2003 to 2005 in seed production in Tongle Township, Fuling District, Chongqing at an altitude of 650 m, a survey found that the damaged plant rate of male parent seedlings without pesticide control reached 50%-70%, while the damaged plant rate of adjacent hybrid mid-season rice was 30%-40%; the damage of the rice stem maggot at the young panicle differentiation stage in seed-production areas is the same as its occurrence in hybrid mid-season rice. If control is inadequate, the damaged plant rate can reach about 70%, causing a yield loss of about 20%.

2.2 Occurrence time and damage symptoms of the rice stem maggot in seed-production fields At the seedling stage of hybrid rice seed production, the rice stem maggot mainly damages early-sown parents, mainly the male parent, II-32A and other parent materials with a long growth period and early sowing; as altitude increases, the occurrence time shows a slight delay. Surveys found that at an altitude of about 300 m, mid-to-late April is the adult egg-laying period, and after the eggs hatch, the initial occurrence of larvae can be seen at the end of April; at an altitude of about 600 m, late April is the adult egg-laying period, and after the eggs hatch, the initial occurrence of larvae can be seen in early May. It mainly damages seedlings in seedling fields, and causes damage to seedlings from the 3-leaf to 8-leaf stage, manifesting as gnawing on the heart leaves, with damaged leaves appearing in rows of holes or thread-like, and the seedlings growing slowly or even stopping growth, significantly prolonging the growth period of the damaged parents and leading to serious failure of flowering-date synchronization. The second damage peak is mainly in mid-to-late June, when hybrid rice seed production enters the young panicle differentiation stage. The larvae damage the growing point, and after the front half of the young panicle is eaten, it becomes degenerated spikelets and cannot form normal grains. After heading, it manifests as about the upper 1/3 of the panicle or part of it having no grains, with only a small number of degenerated whitish branches or deformed small glumes; after grain filling and maturation, the panicle does not bend over, causing serious losses to hybrid rice seed production yield.

3 Control of the rice stem maggot

3.1 Agricultural control of the rice stem maggot In winter and spring, in conjunction with manure accumulation, clear weeds at field edges, ditch edges and hillside edges to reduce overwintering insect sources.

3.2 Chemical control of the rice stem maggot ① Control timing. According to the occurrence period of the rice stem maggot in hybrid rice seed production, the entire hybrid rice seed-production process should focus on 2 periods and 4 specialized control applications. The first period is the seedling stage: the first application should focus on control at the larval hatching stage (late April), and the second application should focus on control at the early larval damage stage (early May); this period is mainly for killing eggs and larvae. The second period is the young panicle differentiation stage: the first application should focus on control at the early young panicle differentiation stage (mid-June), and the second application should focus on control at young panicle differentiation stages 2-3 (late June), mainly to protect the panicle from damage. ② Control pesticides. The field larvae of the rice stem maggot have overlapping generations, making control difficult, so pesticides with strong systemic activity and a relatively long duration of efficacy should be selected. Using 5% fipronil 600-750 mL/ha or 2% Baishiji 750-1050 mL/ha, sprayed with 750-900 kg/ha of water, can achieve a relatively ideal control effect. In hybrid rice seed production, chemical control should be primary and agricultural control supplementary.