The rice rootworm occurred in more than 650 thousand hectares (65% of the cultivated area) in recent harvests, covering all rice growing regions in the state of Rio Grande do Sul and caused productivity losses ranging from 10% to 30%. %, that is, losses greater than 12 bags of rice per hectare. In crops in the state of Santa Catarina, where pre-germinated cultivation predominates, the importance of adults and larvae of Oryzophagus oryzae it is even greater, as it occurs across the entire cultivated area, with losses ranging from 15% to 70%, depending on the cultivar.
Both adults (aquatic weevil) and larvae (root screwworm) cause damage to irrigated rice crops with varying intensity, depending on the degree of infestation, the cultivation system, the cultivar and the sowing time. However, the larval stage, which is submerged in flooded soil, is the most harmful to productivity. The irrigated rice cultivation system favors the occurrence of O. oryzae, which requires the presence of plants and water for its installation and survival. In the pre-germinated cultivation system, infestation and the intensity of damage to adults and larvae are more important due to the early presence of water in the area. When rice is sown, damage already occurs, as seedlings and plants (small) are more susceptible to pest attack.
Efficient management of this pest depends on knowledge of the bioecology of the species, its attack characteristics, the infestation of the areas and the cultivation system. The adoption of Integrated Pest Management (IPM) is essential for reducing damage, as is monitoring control results and improving these practices.
Crop behavior and colonization
During the off-season, the aquatic weevil remains hibernating in crop residues, at the base of grasses, sedges or other plants, as well as in residues in the mud, irrigators, on the edges and/or around crops. After sowing, with the presence of pre-germinated seeds, seedlings and plants, in the presence of water in the rice fields, adults leave their hibernation sites, migrating to the interior of the crop. Adults measure 2,6mm to 3,5mm in length and are grayish in color, with white dorsal spots; They feed on the leaf parenchyma between the veins, forming whitish longitudinal lesions. After approximately three days of irrigation, mating begins and subsequent oviposition begins in submerged parts of the plants (therefore, there must be a plant and water present to begin the attack). Females that have already copulated choose places with deeper water for laying (milder temperatures) or depressions in the land where there are deeper and more permanent layers (crops with greater slopes) where the majority of the larvae subsequently concentrate.
After the embryonic period (egg stage) inside the rice stalk, the larvae feed on the tissues inside the leaf sheath and after 36 hours sink into the irrigation water towards the soil to feed on the roots. They are white in color, are apodous and have six pairs of austoria through which they remove oxygen from the root tissues. They go through 4 instars lasting between 25 days and 30 days and transform into pupa in flooded soil in a waterproof cocoon made of clay, which is attached to a rice or weed root. After 10 days, adults emerge from these cocoons and can start a new generation by invading other rice areas or moving to hibernation sites.
injúrisks and damage toadults and larvae
Regardless of the cultivation system, the greatest losses are caused by larvae, however, weevils cause significant damage to crops in the pre-germinated system, by consuming the radicle and coleoptile, killing the plant shortly after germination. They can attack 100% of the plants, causing the crop to be reseeded. The larvae consume the youngest roots of plants, causing them to rupture, which interferes with nutrient absorption. Typical symptoms are reduced size plants and light green/yellowish leaves. Plants can be easily uprooted. These symptoms are from severely attacked plants, which have a compromised root system and where economic loss has already occurred.
Larval damage is most significant during early root development which coincides with the first generation of O. oryzae, which is considered the most harmful. At this stage the density of larvae is greater and the roots of the rice plants are small and more susceptible. The first generation occurs from September/October, with the beginning of flood irrigation of rice fields. Furthermore, cultivars with a short cycle suffer greater reductions in productivity, as they have a shorter recovery time. With the development of the root system, plants become more tolerant to attack by larvae, coinciding with the second generation of the rootworm, which occurs from January to February (the occurrence of this generation has been disputed). After the initial phase of panicle differentiation, control of larvae is not recommended as there is no response in productivity. Therefore, the most important protection must be given to seeds and seedlings (pre-germinated cultivation) and at the beginning of development, due to their poorly developed root system.
Loss of productivity and Level of Economic Damage (NDE)
The losses in productivity caused by the larvae are quite variable, depending mainly on the degree of infestation and the period of coexistence of the larvae with the crop (moment of control). An analysis of productivity losses caused by the root screwworm, in chemical control experiments, in different cultivars and crops, estimated a loss of 58,1 kg/ha for 1 (one) larva/sample. This loss value was obtained by the difference in productivity of the insecticide treatment with the highest control efficiency and the control without treatment, as well as the difference in the number of larvae per sample of the insecticide treatment with lower infestation and the control. This estimate of the loss caused by one larva per sample will be used in calculating the Economic Damage Level (NDE).
Loss of rice productivity in chemical control experiments of rice rootworm.
| Cultivar | Number of larvae/ sample | Loss/ larva (kg) | Reference |
| BRS 6-CHUÍ | 6,4 | 79,0 | Grützmacher et al (2003) |
| BRS 6-CHUÍ | 20,2 | 64,1 | Botton et al (1998) |
| BRS Taim | 9,2 | 28,6 | Grützmacher et al (2008) |
| IRGA 421 | - | 27,2 | Freitas (2013) |
| BR-IRGA 414 | 4,6 | 110,4 | Martins et al (1996) |
| BR-IRGA 414 | 37,3 | 23,0 | Martins et al (1996) |
| SCS 112 | 17,7 | 42,5 | Prando (2000) |
| SCS 112 | 21,1 | 14,0 | Prando (2000) |
| SCS 112 | 12,8 | 79,6 | Prando (2000) |
| SCS 112 | 20,7 | 86,1 | Prando (2000) |
| Bluebelle | 20,4 | 89,4 | Martins et al (1996) |
| Bluebelle | 22,5 | 36,1 | Botton et al (1996) |
| BR-IRGA 414 | 21,3 | 75,0 | Botton et al (1996) |
| Media | 17,9 | 58,1 |
Cultivar
Number of larvae/
sample
Loss/
larva (kg)
Reference
BRS 6-CHUÍ
6,4
79,0
Grützmacher et al (2003)
BRS 6-CHUÍ
20,2
64,1
Botton et al (1998)
BRS Taim
9,2
28,6
Grützmacher et al (2008)
IRGA 421
-
27,2
Freitas (2013)
BR-IRGA 414
4,6
110,4
Martins et al (1996)
BR-IRGA 414
37,3
23,0
Martins et al (1996)
SCS 112
17,7
42,5
Prando (2000)
SCS 112
21,1
14,0
Prando (2000)
SCS 112
12,8
79,6
Prando (2000)
SCS 112
20,7
86,1
Prando (2000)
Bluebelle
20,4
89,4
Martins et al (1996)
Bluebelle
22,5
36,1
Botton et al (1996)
BR-IRGA 414
21,3
75,0
Botton et al (1996)
Media
17,9
58,1
The choice of rootworm control strategy in rice cultivation is directly related to the cost/benefit of the control measure. For the farmer, the choice of insecticide and the definition of when it will be used is related to four factors: 1- Control cost (insecticide+application); 2- Value of the rice bag and production expectations; 3- Potential for damage from the larvae present in the crop and 4- Efficiency of the chosen insecticide in controlling the larvae. Such factors are components of the NDE equation, which must decide the moment (population density) to control pests. The NDE is a variable number, based on the formula (NDE = [(C / VD) * %M]), where “C” is the cost of control (sum of the insecticide value + application), “V” is the value of kg of rice, “D” is the damage (in kg) caused by the pest and “% M” is the efficiency of the method/insecticide used in control. The efficiency of the method is used as a correction factor for the NDE, as the treatments do not have 100% control.
The producer must decide based on this situation (from the price of insecticide purchased to the value of the bag of rice sold). The calculation of the NDE (value given in larvae/sample), resulting from the formula, shows the maximum limit of larvae that the producer must tolerate in his crop. However, the management decision must be adopted prior to the NDE, defined as Control Level (NC), due to the practical and operational rule determining the aerial application of insecticide.
Estimates of the levels of economic damage from rice rootworm larvae, with variation in the cost of control and the value of a bag of rice, using an insecticide with an average efficiency of 80%. LabMIP/UFSM.
| Cost of treatment (R$ ha-1) | Value of a bag of rice (R$/50 kg) | |||||
| 20,00 | 25,00 | 30,00 | 35,00 | 40,00 | 45,00 | |
| Population of larvae Oryzophagus oryzae / sample* | ||||||
| 50 | 1,7 | 1,4 | 1,1 | 1,0 | 0,9 | 0,8 |
| 60 | 2,1 | 1,7 | 1,4 | 1,2 | 1,0 | 0,9 |
| 70 | 2,4 | 1,9 | 1,6 | 1,4 | 1,2 | 1,1 |
| 80 | 2,8 | 2,2 | 1,8 | 1,6 | 1,4 | 1,2 |
| 90 | 3,1 | 2,5 | 2,1 | 1,8 | 1,5 | 1,4 |
| 100 | 3,4 | 2,8 | 2,3 | 2,0 | 1,7 | 1,5 |
| 110 | 3,8 | 3,0 | 2,5 | 2,2 | 1,9 | 1,7 |
| 120 | 4,1 | 3,3 | 2,8 | 2,4 | 2,1 | 1,8 |
| 130 | 4,5 | 3,6 | 3,0 | 2,6 | 2,2 | 2,0 |
| 140 | 4,8 | 3,9 | 3,2 | 2,8 | 2,4 | 2,1 |
| 150 | 5,2 | 4,1 | 3,4 | 3,0 | 2,6 | 2,3 |
Cost of treatment
(R$ ha-1)
Value of a bag of rice (R$/50 kg)
20,00
25,00
30,00
35,00
40,00
45,00
Population of larvae Oryzophagus oryzae / sample*
50
1,7
1,4
1,1
1,0
0,9
0,8
60
2,1
1,7
1,4
1,2
1,0
0,9
70
2,4
1,9
1,6
1,4
1,2
1,1
80
2,8
2,2
1,8
1,6
1,4
1,2
90
3,1
2,5
2,1
1,8
1,5
1,4
100
3,4
2,8
2,3
2,0
1,7
1,5
110
3,8
3,0
2,5
2,2
1,9
1,7
120
4,1
3,3
2,8
2,4
2,1
1,8
130
4,5
3,6
3,0
2,6
2,2
2,0
140
4,8
3,9
3,2
2,8
2,4
2,1
150
5,2
4,1
3,4
3,0
2,6
2,3
* Reduction in productivity caused by 1 (one) larva/sample (58,1 kg/ha).
For a pre-fixed seed treatment cost when sowing rice, the producer must take into account the expected sales price of the rice. To illustrate, two scenarios were analyzed with a treatment cost of R$ 100,00/ha. Scenario 1: the producer expects to sell a 50 kg bag for R$20,00, so the NDE calculation shows that the number of larvae that can be tolerated is 3,4 per sample. Scenario 2: the producer expects to sell rice at R$40,00/bag, the NDE calculation informs that the decision criteria is more rigorous and he must make the decision to control when the number of larvae is 1,7, XNUMX/sample. Thus, the higher the expected sales value of the rice, the lower the tolerance to rootworm infestation, justifying control with few larvae per sample to avoid greater economic damage.
Real Time of adults and larvae
With the large occurrence in crops and the damage caused, mainly by rice rootworm larvae, the producer must be concerned with monitoring and mainly with the historical record of crop infestation. Monitoring of adults presents different moments in the crop, depending on the cultivation system, with main attention paid to signs of insect feeding on the leaves. In the dry soil cultivation system, weevil monitoring should begin soon after flooding (3 days); or immediately after distributing the seeds in the soil with water or emergence of the plants in the pre-germinated cultivation system, with a thorough inspection of the rice leaves, looking for scrapes on the leaves and injuries to the coleoptile and mesocotyl. This inspection must be carried out at 10 random points in the field, observing the youngest leaves of 20 plants, in each location.
To monitor the larvae, it is recommended to start sampling on the 20th day after the start of irrigation, with the help of a PVC pipe (10cm in diameter) deepening it between 8cm and 10cm into the soil, removing soil and roots. The sample taken must be washed over a sieve and the larvae, due to the difference in density, will float on the surface of the water, facilitating counting. Another methodology used by producers and recommended by some technicians is the direct removal of a clump/plants and washing the roots on a sieve. This technique requires less sampling time, but the sample volume is not known, reducing the accuracy of larvae counting.
Management and control of adults and larvae
Decision-making to control adults or larvae is often carried out even before sowing, using the level of infestation in previous years as a basis. The decision not to control can result in significant productivity losses. The methods widely used to control adults and larvae range from some cultural practices, to the cultivation system and chemical control. Among the latter, seed treatment aims to preventative control of the rootworm. The application of granulated insecticides controls larvae and foliar spraying seeks to control adults.
The destruction of crop residues after harvest and the cleaning of vegetation in irrigators reduces the winter hibernation sites for adults, which reduces infestation in the following harvest. Flattening the soil to standardize the level of water in the crop causes the larvae to spread throughout the area, distributing the infestation and consequently reducing damage. Furthermore, the use of more tolerant cultivars with capacity to recover the root system, from medium to late cycle, and less attractive for adult feeding and oviposition, can be implemented in areas with a history of high rootworm infestations. However, there is little information on the tolerance of cultivars to this pest and there are also few commercial cultivars with this characteristic.
Although cultural practices contribute to reducing pest infestation, in many locations, chemical control with insecticides becomes necessary to avoid productivity losses. Seed treatment is adopted in areas that have a history of occurrence of the pest, as it is a preventive method for the occurrence of larvae, in addition to controlling other soil pests, in a dry soil cultivation system. Seed treatment is the most appropriate method, mainly due to the low amount of active ingredient applied and because it is a targeted insecticide application and not across a total area. However, in the pre-germinated cultivation system, seed treatment presents low efficiency in controlling larvae, leading the producer to seek other control alternatives, such as foliar application to control adults or the application of granulated insecticides to combat pests. larvae.
Soon after the water enters or after the crop emerges, there is a second moment to control O. oryzae, weevils that attack seedlings or plants. This control, based on the amount of leaf scraping caused by the adult when sowing in dry soil or rootlets and coleoptiles cut in a pre-germinated system, is carried out by spraying insecticides 3 to 5 days after irrigation or plant emergence when the Most insects have already migrated to the interior of the crop.
The third moment of control, if seed treatment or foliar spraying had not been adopted (or in the event that they were not efficient), would be the application of granulated insecticides directly into the irrigation water to control the larvae. The decision to carry out this management is based on monitoring the larvae shortly after the start of irrigation and the NDE according to the cost of treatment and the expected sales of the production of each producer's reality.
Finally, it is important to note that the control of rice rootworm presents variable responses depending on the conditions of the location (crop), the year, the cultivation system, which influence its density and control efficiency. On the other hand, the decision to control depends on the importance attributed by the producer to its infestation, but also on the expectation of production and prices for a bag of rice, reflected in the profitability of the crop, but mainly by the cost of treatment. All of these factors affect the occurrence of the plague and also its resurgence and relevance over recent times.
The article appears in issue 194 of Cultivar Grandes Culturas.
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