Management against main pests that attack irrigated rice crops

Rootworm, panicle caterpillar, stem and grain bugs, root aphid and stem borer are among the main pests that attack irrigated rice crops. Correctly identifying these insects, determining the level of economic damage and using chemical control at the correct time and in a rational manner are the path to sustainable management of cultivation areas.

The incidence of insects in recent periods has worried producers, as in addition to high populations, new species are attacking irrigated rice crops. The producer must be concerned about the main pests, for which strategies can be adopted throughout the crop cycle, due to the economic damages. Knowledge of the history of pest occurrence in the area is of great importance for the implementation of control methods. Therefore, the producer must be aware of the occurrence of insects, so monitoring must be adopted to estimate the population.

IMPORTANT ASPECTS IN PEST MANAGEMENT

Identification and period of occurrence

Knowledge of the impact of pests on irrigated rice production and the conditions that favor their increase are important factors in their management. First, it is necessary to know and know how to identify the main pests, the damage caused and the critical stage of the plant for insect attack. There are also secondary insect species that are unlikely to cause damage to the crop.

Level of economic damage

It is the population density of the pest that causes a reduction in grain yield, equal to or greater than the estimated cost of control. The simple presence of the insect in the crop does not necessarily determine control. This should only occur when the population reaches levels that could cause damage to the crop.

Real Time

Monitoring is the correct way to determine the existing population of the crop through sampling that must be carried out weekly. The number of samples and the size of the collection area will determine the accuracy of the method. Therefore, the greater the number of samples, the greater the monitoring accuracy. Determining the insect population can be done by collecting at least 30 representative samples from the area.

natural enemies

Beneficial insects are important due to their constant presence and relative abundance. Measures to preserve them, such as leaving refuge areas next to the crop after harvest, where they can shelter in unfavorable conditions during the low off-season temperatures, will guarantee their survival. Among the main species of natural enemies, spiders stand out. In rice crops in Rio Grande do Sul, the main species is Alfaida venilae, which corresponds to 80% of the population. Spiders do not damage plants, they have the ability to quickly increase the population and in the case of population explosions, cannibalism can occur. Dragonflies, earwigs and ladybugs are also important predators, as they feed on moths, eggs, pupae, small caterpillars, mites and aphids.

MAIN PESTS

Rootworm, Oryzophagus oryzae

It is the most important pest, as it occurs in all regions and attacks more than 65% of the sown area of ​​Rio Grande do Sul. In addition to the species O. oryzae, others were found in the state of Rio Grande do Sul, such as: Lissorhoptrus bosqi, carinirostris, lepidus e tibialis. In this last species, adults are larger than the others.   

The insects hibernate inside or outside the crop, but from September onwards, with the increase in temperature and irrigation, the adults are attracted to the crop, lasting until December. Entering the crop, they first feed on the leaves, leaving white scars longitudinal to the main vein and then the females lay their eggs on the stalks. Afterwards, the larvae emerge, feeding inside the leaf sheath and then moving to the roots. These have 6 pairs of austoria, whose purpose is to remove oxygen from the roots. Larvae have an average cycle of 4 weeks. 20 days after irrigation, the larvae begin attacking the roots, causing extensive damage. A second infestation generally occurs 50 days after irrigation, and as the larvae prefer new roots, damage may occur.

Monitoring the larvae is important and should be started 20 days after irrigation, examining the roots. After, plucking 15 plants/sample, placing them in a bucket of water and, when shaken, the larvae will appear on the surface and are easily contacted. Samples must be collected in areas favorable to the occurrence of larvae, in crops sown first or with higher water depths.

The characteristic symptoms are smaller plants and damaged root systems. The roots appear to be little destroyed, but when removing the soil it appears that the central part is the most attacked.

The largest infestations of larvae occur from 35 to 45 days after the start of irrigation, causing damage of around 10%. Early cycle cultivars are the most damaged. Each larva/sample, on average, can cause a reduction of up to 1,5%.

Panicle caterpillar, Pseudaletia sequax and P. adultera

It is one of the main pests, it is distributed throughout Rio Grande do Sul and has occurred in more than 45% of the crops. There are two species: Pseudaletia adultera and P. sequax, and the adulterates accounts for more than 70% of the population. Females lay their eggs on leaves and stems, laying up to 1000 eggs. Afterwards, the caterpillars appear and attack the plants, on average, for 28 days. In the first stages the caterpillars feed on the leaves and in the remaining stages they cut the panicles. The  differentiation of the caterpillars of these species is viable from the fourth instar onwards, with  the P. adultera are dark and P. sequax, pink. 

The critical period for the panicle caterpillar is from flowering to harvest. During the day the caterpillars are sheltered in the lower part of the plants, rising at night to attack the panicles.

Monitoring must be carried out in the critical phase. Sampling is recommended in which the plants are opened to check the occurrence of caterpillars or whether there is part of the panicle lying on the ground. When 2 caterpillars/m² are found, control measures must be adopted, as when these insects occur in high populations, losses in productivity can reach 20%. 

After harvesting, the caterpillars hibernate inside the crop, taking shelter in the stubble, in higher areas. Even with low temperatures in June, caterpillars were found in the rice straw. 

Stem bug, Tibraca limbativentris

This insect is one of the main pests, it has attacked more than 40% of crops and occurs in all regions of Rio Grande do Sul. Due to its annual occurrence, high populations in recent harvests, and because it is distributed throughout the area, it has worried producers. 

From September onwards, with the increase in temperature and photoperiod, the adults leave their hibernation sites and head to the fields. At first the adult population is low, but with the laying of up to 500 eggs per female, there will be a large population increase.

Stink bug damage occurs in two stages of the plant: during tillering, they attack the stem, where a dark colored spot is observed, due to the strangulation of the stem, causing the death of the central leaf, a symptom called "dead heart". In the reproductive phase, it causes the formation of white panicles or with a large number of sterile grains. Each stink bug/m² can cause losses of 2% in grain yield.

Monitoring must be carried out weekly from the beginning of tillering until flowering. As the stink bug has a habit of remaining on the lower part of plants, the infestation is generally not observed. Then sampling must be done, preferably in the mud, opening the plants to locate the insects. In many unmonitored crops, the stink bug attack was observed late, when white panicles appeared or at harvest when examining the bulk carrier and during pre-cleaning, due to the high incidence of insects.

With the rice harvest, stink bugs hibernate from the end of April in areas within or close to the crop that are not subject to flooding, sheltering in crop residues, weeds, the main host being donkey tails, Andropogon bicornis. In sugarcane, up to 10 stink bugs/plant were found sheltering between the ligule and the stalk. During hibernation in several crops in Rio Grande do Sul, insects attacked by fungi were collected, highlighting the Beauveria bassiana.

Grain bug (Oebalus poecilus and O. ypsilongriseus)

The grain bug is a chronically occurring species and is distributed throughout the state of Rio Grande do Sul. Adult insects, after leaving their hibernation sites, can be found in fields, pastures or on weeds close to crops, looking for seeds for food. The main hosts are: Echinochloa spp, Paspalum spp, Brachiaria spp and Digitaria spp. Subsequently, the females head towards the crop, laying eggs on leaves, rice grains or weeds, possibly surrounding them completely and, after 7 days, the nymphs emerge. Even though they occur in outbreaks, the insects move and lay their eggs in various parts of the crop.

On days with high temperatures, lots of light and wind, insects take shelter in the central part of the plants. Sampling should be carried out in the late afternoon, when stink bugs are found in the upper parts of plants. Monitoring must be carried out during flowering, avoiding losses.    

Damage is caused by nymphs and adults, with the formation of sterile panicles in the milky stage. In the final stage, the grains are weakened and break when processed. In addition to direct damage, insect bites can transmit fungi that cause stains on grains. For each insect/m² there is a 1% reduction in grain yield.

Root aphid, Rhopalosiphum rufiabdominale

This insect is one of the main insects in the Western Border region of Rio Grande do Sul, also occurring in the Central Depression and Outer Coastal Plain.

In the adult form, it has a dark green abdomen with an orange-red area. The species is characterized by having antennae with 5 articles. Females are winged or wingless, with the latter predominating. Aphids overwinter on weeds, especially the rosette, but are also found on ryegrass, donkeytail and barnyard grass. As they attack the crop before irrigation, forming colonies on the roots, monitoring must be adopted at this stage.

Aphids, as they are dark in color, are difficult to observe, as they are confused with the color of the soil. Sampling must be done by uprooting the plants, and by examining the roots the insect can be detected. Also, when placing light-colored paper and shaking the plant, the insect will fall along with the soil and soon after that the aphids will begin to move, making them easier to identify.  

The damage caused by aphids to the roots is observed in plants with chlorosis at the tips of the leaves, wrinkled, red-orange in color, later dry plants. The largest infestations occur in the mud, reducing the initial stand and then causing the death of the plants.

Stem borer, Diatraea saccharalis

The stem borer has increased its population and occurs every year, attacking crops in the Western Border region. Adults are nocturnal and are characterized by having yellowish forewings, with two transverse striations in the apical third. Females perform postures in groups, on the upper part of the leaves; then the caterpillars emerge, piercing the stems of the plants and lodging inside. They go through 6 stages and have an average cycle of 30 days.

Stem borer monitoring is done by identifying symptoms and drilling holes in the stems. The rammed earth is the place on the farm with the highest stem borer infestation.

Damage in the vegetative phase occurs 30 days after irrigation and is caused by the death of the central leaf. During the reproductive phase, white panicles form. Caterpillars also attack the forming grains.  

The damage caused by the stem borer has caused a 10% reduction in productivity.

There is no registered insecticide for controlling stem borer in rice.

Driving

The incorporation of stubble and the elimination of weeds in the streets, mud and crop edges will help reduce the population. The existing grass in these areas should only be cut, in order to keep natural enemies out.

Crop rotation, In addition to improving productivity, it helps control pests. In this system, a crop must be sown that the insect does not feed on. In rice areas this measure is being adopted successfully, using soybean crops.

Correct irrigation requires a sheet of water covering all parts of the crop, as dry areas favor the development of insect pests. Management to reduce root aphid damage involves placing a high water slide for a week.

Chemical control should be adopted when population levels may cause losses. When choosing the product, it is important to apply registered, efficient and low-toxic pesticides. The use of non-selectives results in the elimination of beneficial insects and it may occur that an unimportant species becomes a primary pest. Never apply the same product for several years, as it may cause resistance.

The sequential application

The use of insecticides applied preventively in mixture with fungicides is incorrect and is often not efficient, which can increase production costs and affect natural enemies. Managing correctly does not mean using products more frequently and without technical criteria.

 

Jaime Vargas de Oliveira, UNITEC; Danielle Almeida, IRGA; Lidia Fiuza, IRGA; Paulo Luciano Pereira de Siqueira, PAVA Consultoria

Article published in issue 207 of Cultivar Grandes Culturas.