Control of fall armyworm in corn

The most destructive pest of corn crops is the fall armyworm, Spodoptera frugiperda is capable of reducing productivity by more than 50% and has become a concrete threat to hybrids in recent years Bt, which in some cases require the use of insecticides to complement control. Quantifying the costs of this technology when associated with the application of insecticides and identifying which ones are still effective against the insect are crucial issues.

Corn is cultivated on approximately 16 million hectares per year in Brazil. In most states, cultivation occurs in two harvests per year, called the 1st and 2nd harvest, and in recent years the 2nd harvest has surpassed the 1st in terms of cultivation area and production. Furthermore, in regions with irrigation, there is also an additional harvest for the production of seed corn. However, in some states in the Southern Region (Rio Grande do Sul and Santa Catarina), due to the harsh winter, the 1st harvest is the main season for corn cultivation and production. However, regardless of the place of cultivation, the fall armyworm, Spodoptera frugiperda (J.E. Smith, 1797) has been considered the most destructive insect pest of corn crops, being capable of reducing productivity by more than 50%.

the management of S. frugiperda in corn was carried out mainly through chemical control, but from 2007 onwards, with the commercial release of corn technologies expressing insecticidal proteins from Bacillus thuringiensis Berliner (Bt), a new control tactic for the pest emerged. Currently, Bt corn is cultivated on approximately 12,5 million hectares per year in Brazil. However, within a few years, a breakdown in resistance and a consequent reduction in the effectiveness of the Cry1F and Cry1Ab proteins for the fall armyworm has been reported (Farias et al., 2014; Omoto et al., 2016). Control failures S. frugiperda by some Bt technologies have been noticed, making it necessary to use insecticides to complement control. In recent harvests, the use of insecticides on Bt corn has increased considerably in Brazil, including in Rio Grande do Sul.

BIOTECHNOLOGIES AND INSECTICIDES EVALUATED         

To support the management of S. frugiperda in corn, the team from the Integrated Pest Management Laboratory (LabMIP) at the Federal University of Santa Maria (UFSM) evaluated the effectiveness of Bt corn technologies and their association with insecticides in controlling fall armyworm. The studies were conducted in Santa Maria, Rio Grande do Sul, in a field with natural infestation of the pest, during the 1st harvest 2015/2016.

Nine corn hybrids were evaluated, eight Bt and one non-Bt technologies and six insecticide spraying programs and the control treatment (without spraying). Tests were carried out between all biotechnologies × all chemical treatments.

DAMAGE ASSESSMENT

Damage assessments of S. frugiperda in Bt and non-Bt corn, they were carried out every five days, on the three central leaves of the “cartridge”, of 10 consecutive plants in the central line of each plot. The number of plants with damage was counted (converted into percentage of plants with damage) and a damage grade was assigned, according to the Davis scale (0 - no visual damage to 9 - severe damage to the cartridge) (Davis et al., 1992). Insecticide spraying was carried out when 10% of the plants, in each sampling, showed damage. >3 (Davis Scale). The volume of spray used was 200 l/ha. 

RESULTS

There was a high percentage of plants with damage from S. frugiperda in Agrisure TL, Herculex and Optimum Intrasect corn, varying between 55% and 60%, being similar to those occurring in non-Bt corn (68% of plants with the presence of damage). For these technologies, the average damage score (Davis Scale) ranged from 2,1 to 2,9 (Figure 1). In these Bt and non-Bt corn technologies, there was a need for one to three insecticide sprayings to complement the control of S. frugiperda and avoid significant economic losses. In contrast, in YieldGard VT PRO (20%), YieldGardVT PRO 3 (14%) and PowerCore (14%) corn there was similarity in the percentage of plants with damage. However, the damage was less significant (average damage score below 0,5) than that observed in the previously mentioned hybrids. On the other hand, in Agrisure Viptera and Agrisure Viptera3 corn, S. frugiperda were insignificant (0,5% of plants with the presence of damage). In corn YieldGard VT PRO, YieldGard VT PRO 3, PowerCore, Agrisure Viptera and Agrisure Viptera 3 there was no need to use insecticides to complement pest control.

Among the insecticide treatments tested, ospinetoram alone, in the 1st and 2nd spray, and the insecticides: methomyl + chlorantraniliprole (1st spray) and indoxacarb (2nd spray); lambda-cyhalothrin + lufenuron (1st spray) and lambda-cyhalothrin + chlorantraniliprole (2nd spray) were the most effective in controlling surviving caterpillars of S. frugiperda in Bt and non-Bt corn (Table 3). The combination lambda-cyhalothrin + lufenuron (1st spray) and lambda-cyhalothrin + chlorantraniliprole (2nd spray) was the one that presented the best cost-benefit (control).

In addition to evaluating the performance of Bt corn and insecticide technologies in controlling S. frugiperda, control costs were objectively estimated (Table 3). The final cost of controlling S. frugiperda in Agrisure TL corn, Herculex, Optimum Intrasect and non-Bt corn ranged from R$ 494,00/ha (non-Bt corn) to R$ 734,00/ha (Agrisure TL corn), with these numbers being directly influenced by the cost seed initial, which is lower for non-Bt corn (Table 3). In contrast, the percentage of damaged plants in corn YieldGard VT PRO, YieldGard VT PRO 3, PowerCore (between 14% and 20%), Agrisure Viptera and Agrisure Viptera 3 (between 0,1% and 0,5%) were lower, dispensing with insecticide applications to control S. frugiperda, with control cost being restricted to the Bt hybrid used (Table 3).

In general, the productivity of Bt corn technologies (with or without insecticides) was relatively higher than non-Bt corn. There was great variation in productivity between Bt corn technologies, which occurred due to the presence or absence of crop damage. S. frugiperda, but also largely due to the influence of genetic material (hybrid), which naturally varies its productive capacity, which is reflected in the value of the seed. 

In summary, several corn technologies failed to control fall armyworm, which demonstrates that there will be a considerable increase in the use of insecticides on Bt corn to complement the control of the pest. However, choosing Bt hybrids with better efficacy against fall armyworm may be an alternative to minimize the need for insecticides and, consequently, reduce control costs. Furthermore, there is a need to monitor the presence of surviving caterpillars in Bt corn and establish action levels for the use of insecticides, if necessary. However, the adoption of other management practices, such as refuge areas, should also be considered for the management of S. frugiperda in corn.  

Leonardo Burtet, Jerson V. C. Guedes, Oderlei Bernardi, Adriano A. Melo, Maiquel Pizzuti Pes, Thiago Strahl, Ericmar A. dos Santos and Frederico Hickmann, Federal University of Santa Maria

Article published in issue 210 of Cultivar Grandes Culturas.