Stem borer control measures in irrigated rice

Considered a secondary species, the stem borer has caused concern to irrigated rice producers in Rio Grande do Sul due to the increase in the insect's population and also the damage caused to the crop. Losses of up to 410kg/ha have already been observed in experimental studies in the last harvest. Its control involves several measures such as elimination of weeds, destruction of stubble, balanced fertilization and preservation of natural enemies.

Phytophagous insects that attack irrigated rice crops are considered important when they occur more frequently and cause damage. Losses in production are high, as when they infest crops from sowing to harvest, the losses are significant and reach up to 37%.

In recent harvests, the incidence of insects has worried producers, as in addition to the increase in populations, a secondary species has occurred more frequently and caused losses. This is the case of the stem borer, Diatraea saccharalis, which is expanding. Due to favorable climate conditions for the insect and probably the reduction of predators, the producer must be attentive and monitor his crops.

The stem borer currently occurs in several rice-growing regions of Rio Grande do Sul, but the main infestations are found in the Western Fronteira region, where it showed a different behavior when attacking the grain. Also in the Inner Coastal Plain and Outer Coastal Plain regions, many crops were attacked.

Biology

The nocturnal adults are sheltered in plants during the day and are characterized by having straw-yellow forewings, with dark spots and lighter hindwings. The females perform postures on the leaves, grouped together, with a scale-like appearance. After seven days, on average, the caterpillars emerge, which go through six stages; In the first instar they remain in the leaves, then they penetrate the stem, and can remain until the pupal stage. The caterpillar stage is approximately 30 days. The pupal stage follows, lasting approximately ten days, and later the adult insects emerge.

It is considered one of the main pests of sugarcane, however it attacks other economically important crops such as corn, sorghum, wheat and pastures. This diversity of hosts makes it easier for the pest to remain in crops and spread.

Real Time

Monitoring must occur during the plant's greatest susceptibility stage, through the presence of adults and postures. As the insect is present practically throughout the entire crop cycle. It is recommended to obtain samples from the stem elongation phase until close to harvest. Monitoring must be carried out by opening the plants to check the incidence of the insect.

The number of samples and the size of the collection area will determine the efficiency of the method. The more samples, the greater the precision. Samples must be obtained at random points, traveling across the crop in a transversal direction. It is recommended to carry out at least 20 samples to 30 samples, starting 10m from the edges of the crop and analyzing an average of ten stalks per point.

Symptoms and harm

Due to the habit of remaining inside the plant, symptoms are observed when damage has already occurred. They are often confused with those caused by the stem bug (Tibraca limbativentris). In the case of attack by the stem borer, the symptom can be identified by the occurrence of a hole and, generally, by the presence of caterpillar residue.  

The first instar caterpillars stay under the leaf sheath causing the outer part to yellow. However, the attack of the caterpillars on the culms in the vegetative phase generally begins 30 days after the start of irrigation, causing the death of the central leaf, a symptom known as “dead heart”. These parts of the plants, if pulled, come loose easily.

In the panicle emission stage due to the penetration of small caterpillars into the upper internode, partial or total drying of the panicles occurs, with the symptom known as “white panicles”. The caterpillars can leave the stems and attack other nearby plants.

In addition to the caterpillar's habit of attacking the stem of host plants, attacks by the stem borer directly on rice grains have been identified in the Western Frontier region. During this period, the caterpillar feeds on the forming grain, being able to attack several grains per panicle.

The damage caused by the caterpillar when attacking stalks and grains, even though it is not a widespread occurrence in crops, has caused losses in productivity, reaching a 10% reduction.

In experimental studies carried out in the last harvest, in the municipality of Alegrete, there were losses in grain yield of 410kg/ha, or more than eight bags/ha, demonstrating the damage caused by this insect pest to irrigated rice crops in the state.

Control measures

Since the damage caused by stem borer caterpillars occurs inside the stem, where control is difficult, several important methods must be adopted.

Elimination of weeds

Host plants in irrigation canals, streets and crop edges must be eliminated to prevent insects from taking shelter during the hibernation period. However, the existing grass in these locations must be cut and not eliminated, to maintain natural enemies.

Destruction of the remnant

Eliminating stubble after harvest, through incorporation using a harrow or knife roller, will help reduce the insect population. The highest parts of the crop containing straw are favorable places for this insect to shelter.  

Fertilizing

A balanced fertilization will form stronger plants, a greater number of tillers, more developed roots, better withstanding pest attacks. 

Favor the action of natural enemies

Natural enemies play an important role, as they reduce pest populations, making them more stable. An increase in the population of predators can be encouraged by avoiding the burning of stubble and areas close to the crop. Among the main species of natural enemies, spiders stand out, which play an important role in controlling insects due to their constant presence and relative abundance during all phases of culture. During the day they stay at the bottom of the plants, rising in the late afternoon to capture insects. Other predators such as dragonflies, earwigs and grasshoppers are also important, as they feed on adult insects, eggs and small caterpillars.

The different species of natural enemies of irrigated rice are of great importance in insect management practices, which is why they must be monitored and preserved.

Jaime Vargas de Oliveira, Unitec; Danielle Almeida, Irga/EEA; Luciano Araujo Freitas, Igor M. de Oliveira Pias, Caal

Article published in issue 197 of Cultivar Grandes Culturas.