Occurrence Patterns and Control Methods of the Rice Water Fly in the Aksu Region

The rice water fly (Ephydra macellaria Egger) belongs to the order Diptera, family Ephydridae. It occurs extremely severely in newly reclaimed salinized rice areas and is one of the major pests in rice production in the Aksu region. The insect gnaws on primary and secondary roots, causing poor development of rice seedlings or floating seedling death. Surveys of rice in the Aksu region from 2002 to 2003 found that yield losses in lightly infested fields reached 5.8%-8.1%, and in heavily infested fields reached 40.5%-56.1%, seriously affecting rice production in this region. The authors conducted investigations and research on the morphological characteristics, life habits, occurrence patterns and control methods of the rice water fly.

I. Morphological Characteristics

The adult body length is 6-8 mm, wingspan 8-10 mm, body grayish-brown to blackish-gray, head lead-gray. The compound eyes are densely covered with short black hairs, with a golden-green sheen on top, and the dorsal thorax is purplish-blue. The egg is 0.5-0.7 mm long, nearly round, initially milky white, later turning yellowish-white. The larva has 11 segments, earth-gray, with a pair of pseudopods on each of abdominal segments 4-11, eight pairs in total. The body surface is smooth, with bristles. The mouth hook is dark brown, forked at the posterior end. The pupa is a coarctate pupa; at eclosion the front end of the pupal shell splits in a ring, belonging to the ring-splitting type. Length 8-10 mm, width 2-3 mm, body with 11 segments, cylindrical, initially yellowish-brown, later turning yellowish-brown or brownish-brown. During the process from larva to pupa, the pseudopods on tail segments 9-11 form ring hooks suitable for attachment to the roots, stems and leaves of rice and weeds.

II. Distribution and Damage

The regions where damage by the rice water fly has been reported in China are: **, Ningxia, Hebei, Liaoning, Gansu, Shaanxi, Shandong, Inner Mongolia and other provinces. In this region, its hosts include reed grass, triangular bulrush, barnyardgrass, wild rice, crabgrass, foxtail, Equisetum ramosissimum, cyperus and other gramineous weeds. The rice water fly is a major pest of rice seedlings in saline-alkali land. Its larvae bore into and feed on newly sprouted rice seeds, causing seed rot, and gnaw on the primary and secondary roots of rice, sucking sap and nutrients, causing floating seedlings. In addition, after the larvae pupate they become lodged on rice roots, seriously affecting the normal development of rice. In the Aksu region, the rice water fly mainly causes damage during the two stages of rice sprouting (white tip) and needle standing.

III. Occurrence Patterns

1. Life History

The rice water fly has four generations per year in the Aksu region, with each generation lasting about 40 days, with overlapping generations. It overwinters as adults in cracks of large soil clods, under salt crusts at field ridge edges, and under the leaf litter of white poplar, Russian olive and weeds. In late March to early April of the following year, when the temperature rises to 14-15°C, the adults first become active on the water surface of alkaline areas at rice field edges and stagnant pools, feeding on humus. After water is introduced into the seedling field, the overwintering generation adults lay eggs on debris and floating objects in the rice field; the larvae first feed on humus and then damage rice. The sprouting (white tip) and needle standing stages of rice are the main damage periods. By the tillering stage of rice, the rice root system has firmly established in the soil and the plants are robust, so they are no longer damaged, and the pest then reproduces on weeds at the edges of rice fields.

2. Main Habits

Adults of the rice water fly have strong phototaxis; black light lamps with a wavelength of 3650 angstroms can attract large numbers of rice water fly adults. Adults like to gather and rest on the dirty foam layer on the water surface and move about on sewage surfaces; wherever stagnant water accumulates, adults are active. They are especially active when the temperature rises, chasing each other and mating, feeding on decaying organic matter floating on the water surface and laying eggs on floating objects. Where there is more floating matter on the water surface, more adults are attracted and gathered, more eggs are laid, and larval density is also greater. Because floating matter is often blown by wind to the field edges and ridge areas, the larvae and pupae are mostly distributed at the edges and corners of rice fields. They occur more in stagnant pools, alkali drainage ditches, newly opened wasteland and heavily saline-alkali rice fields. Damage is often severe at field edges, and timely control is also key.

IV. Relationship Between Occurrence and Environment

1. Relationship Between Temperature and Occurrence

The occurrence of the rice water fly is closely related to temperature. The developmental zero temperatures for each stage of the rice water fly are: adult 9.3°C, egg 10°C, larva 12.7°C, pupa 12.4°C. The effective accumulated temperature required for a full generation is 302.4°C.

2. Influence of Different Soil pH Levels

This region has a high degree of soil salinization. The rice water fly occurs more in stagnant pools, alkali drainage ditches, newly opened wasteland and heavily saline-alkali rice fields. The rice water fly prefers saline-alkali conditions and only lives in water with a pH of 7.5-9; it cannot survive in environments with a pH greater than 9.

3. Influence of Land Leveling

The occurrence and damage degree of the rice water fly are greatly affected by cultivation level. Level plots have light occurrence; conversely, damage is severe.

4. Influence of Natural Enemies on the Occurrence of the Rice Water Fly

According to observations, the main natural enemies of the rice water fly in this region include frogs, fish, ants, ground beetles, spiders, birds, etc. Among them, frogs are the main controlling natural enemy; adult frogs feed on adult rice water flies, and their larvae, tadpoles, feed on rice @#@261@#@.

V. Control Methods

1. Agricultural Control

Strengthen field management: level the land, clear away the previous crop, and regularly remove floating matter from the water layer.

Soak the fields and wash out alkali, drain and irrigate frequently, and keep the field water clean. For newly opened wasteland and heavily saline land, soak the fields and wash out alkali to lower the pH. Frequent drainage and irrigation, turning stagnant water into flowing water, can not only wash away salt and alkali and create an environment unfavorable for the rice fly maggot, but also wash away floating matter on the water surface, keeping the water surface clean.

Cultivate strong seedlings and select varieties that grow quickly at the seedling stage, have firm and upright leaves, and are strongly cold-tolerant.

2. Biological Control

Protect and utilize natural enemies for control. In this region, carbofuran is of low toxicity to frogs and has good control efficacy.

3. Chemical Control

In early spring, before water is introduced into rice fields, control should be carried out in stagnant pools and alkali drainage ditches at the edges of rice fields to eliminate early breeding sites, using decis or 80% dichlorvos emulsifiable concentrate.

In the later growth stage, granular formulations or poisonous soil can be applied to rice fields for control: 2-3 kg of 3% carbofuran granules per 667 sq m. Alternatively, 150 g of 80% dichlorvos emulsifiable concentrate can be diluted with an appropriate amount of water and mixed with 20-25 kg of fine soil for broadcasting.