Rice Stem Borer, a Pest with 40 Years of History in Iran

Forty years have passed since Professor Albert German reported and confirmed the presence of the rice stem borer in northern Iran. During this period, the population of this moth from the Pyralidae family has had its ups and downs, but it has always been considered one of the key pests of rice in our country. In past years, numerous research has been conducted to identify and control this pest. Today, in northern Iran, various agricultural, chemical, and biological control methods are used to keep the population of the rice stem borer low. The present article provides a brief overview of the characteristics of this pest and the various control methods.

The rice stem borer, Chilo suppressalis (Lep.:Pyralidae), was first identified and reported by Walker in 1863. This pest has been reported in rice fields of Southeast Asia, in countries such as Japan, Philippines, Indonesia, Thailand, India, Bangladesh, as well as in Spain, Portugal, and Egypt. In Iran, damage from this pest has also been reported in Isfahan province, in addition to the northern provinces of Gilan, Mazandaran, and Golestan.

Morphology and Biology
The body length of the adult insect is reported to be 10-13 mm, and the body width with wings spread in females is 35 mm and in males is between 20 to 23 mm. The general body color is yellow and sometimes tends to light brown. On the front wings, there are several silvery spots and at the edge of each wing, 5 brown spots. The underwings are yellowish-white and the edges of the wings are equipped with cilia. The eggs are initially seen as whitish-cream colored, which turn gray when hatched. The length of full-grown larvae is 26 mm and the width of the head capsule is 2.5 mm. On the dorsal surface of the larvae, five longitudinal stripes of bright purple are visible. The color of the pupae is light brown, with their length recorded between 11-13.5 mm and their diameter 2.5 mm. Pupae are formed inside rice stems in the field during spring and summer.
In terms of host range, this pest is oligophagous, meaning it can feed on grasses in addition to rice. Egg-laying in rice fields occurs during the rice growth period (tillering and main field) on rice plants. In Mazandaran, the emergence of the first moths is observed in late Farvardin and early Ordibehesht. The first generation moths (overwintering) lay their eggs on the upper or lower surface of the upper leaves, while egg-laying by the second generation moths has been reported on the lower leaves or on the stems near the leaves. First-instar larvae initially feed on the leaf parenchyma and then enter the stem by creating a hole from the leaf sheath. The larval cycle length has been recorded as 21 to 37 days. After the pupae are formed inside the rice stems, it takes 7 days for the second generation (spring) moths to emerge from them. The maximum population of this generation's moths is seen in late Tir and early Mordad. Therefore, the first generation appears from late Farvardin and is seen until mid-Tir. The next generation is active from mid-Tir to late Mordad, and traces of the third generation are visible from late Mordad to late Mehr. Under favorable conditions, each stem-borer generation usually lasts 45 to 50 days.
After harvesting, they migrate to weeds. 22 weed host species have been identified for this pest. Some of the host weed species of this pest include:
This pest overwinters as a full-grown larva inside the stubble remaining after harvest and also in the stems of intermediate host weeds on the edges of fields, including Sorghum halepense, Lolium perenne, Setaria viridis, Avena fatua, Phragmites australis, Panicum miliaceum, Echinochloa crus-galli, Digitaria sanguinalis, Bromus japonicus, and others.

Damage
The damage caused by this pest to rice plants can be described as an attack on the rice stem. The damage symptoms are visible in two forms; if a young rice plant is attacked by the pest, the central leaf (young shoot) turns yellow and gradually dries out. This form of damage, which is named "dead heart," is usually observed in the first generation of the pest during the vegetative stage of rice. In this case, the plant's reaction is to produce tillers to repair the damage. However, if the attack coincides with the emergence of panicles and flowering of the plants, which corresponds to the damage of the second generation of the pest, the plant cannot compensate for the damage and no grains are formed in the panicles, causing them to turn white, a condition commonly known as "white head." In this case, either no grains are formed at all, or the rice panicles remain white, thin, light, and very brittle. During threshing, the grains break easily, resulting in a significant decrease in the product's value and marketability. At harvest time, the stems infested by stem borer larvae break with wind and fall to the ground. These stems have many small holes, inside which numerous larvae can be seen. Thus, if damage occurs at this stage, it is impossible to repair, and the economic injury threshold of the rice stem borer at this time is lower than that of the first generation. Considering the coincidence of the emergence of different generations of this pest and the phenology of different rice varieties, it can be said that the rice stem borer causes damage in at most two generations in early and medium-maturing varieties, while in late-maturing varieties, this damage extends to three generations.

Chemical Control
According to the study by Khosroshahi et al. (1354), the yield from the application of granular pesticides was better than that of emulsifiable pesticides. Khosroshahi et al. (1358) showed that two applications of granular Diazinon can preserve 60% of the crop.
In older sources, the use of 10% and 5% granular Diazinon, Padan (Cartap) 4%, and Furadan granule 3% has been recommended. The first granular application is made 21 days after transplanting, and the next two applications are repeated at 3-week intervals.
Resistance Management:
To prevent resistance from developing in the population of rice stem borer, the application of different pesticides according to the following formula is recommended.

Agricultural Control

1- Collection and burning of infested leaves containing eggs during seedling stage
2- Cultivation of other crops after rice harvest: According to the report of Mostafipour and Heydari (1988), infestation in fallow fields was higher than in fields cultivated with clover. The main factor for the transfer of infestation from one year to the next is the host weeds at the edges of the fields
3- Close harvesting: Harvesting the crop from the base near the crown to reduce the number of overwintering larvae in the field
4- Prevention of pest transfer to storage: Drying the bundles in the field and threshing them with a thresher in the field
5- Leveling the field immediately after plowing
6- Uprooting and burning weeds at the edges of the fields including grasses, wheat-like grasses, reeds, canary grass, bindweed, reedmace, alfalfa, wild oats, foxtail, and sorghum
7- Use of early-maturing varieties or preparation of seedlings under plastic
8- Use of resistant varieties: In resistance of rice varieties to rice stem borer, both physical and chemical properties of the plant are involved (Das, 1997). In various studies, the variety 'Nemat' was reported as the most resistant variety due to the high amount of silica in its stem (13.9%), and the varieties 'Khazar' and 'Bi Nam' showed moderate resistance to the first and second generations of rice stem borer.
b- Trapping adult moths with light traps: Using yellow light traps, adult insects can be collected and their egg-laying can be prevented.

Pheromone Control
Saeb et al. (2002) investigated the attractiveness of the synthesized internal pheromone of rice stem borer moth with the formula Z-13-octadecenal, Z-11-hexadecenal, Z-9-hexadecenal at ratios of 1:5:1 with doses of 0.5, 1 and 2 mg, compared to the external pheromone in the cities of Rasht and Sowme'eh Sara. They concluded that the internal pheromones were more effective than the external pheromone and can be used in the integrated management of rice stem borer.
To control rice stem borer using this method, by installing pheromones at a certain height and specific area, the environment can be saturated with pheromone compounds, confusing the male moths and thus preventing mating between male and female insects. The number of fertilized eggs is reduced and the pest population in the field is significantly decreased.