Integrated management of fall armyworm in corn

Leaves, ears, neck and stalk of corn plants are among the targets of the fall armyworm, Spodoptera frugiperda, one of the main crop pests, responsible for damage of up to 60%. To contain the damage caused by this insect, integrated measures are necessary, adopted throughout the year, which include desiccation before sowing, seed treatment with insecticides and the use of technology Bt.

Corn is one of the most important crops in the world. To obtain high productivity, it is necessary to protect plants against pest attacks. Compared to other crops, the population of plants in corn is smaller, meaning that the loss of leaf area or plants, due to errors in phytosanitary management, has a greater impact on grain productivity (Cruz et al., 2010).

One of the main pests of corn is the fall armyworm (Spodoptera frugiperda). Its damage can reach up to 60%, depending on the hybrid, sowing time and stage of crop development, with flowering being the critical period (Cruz el al., 2010; Carvalho, 1970). The larval period of the fall armyworm varies from 12 days to 30 days, depending on the temperature, and it is during this period that damage to corn plants occurs. The caterpillar's color can vary from gray to brown to green, with six instars, reaching 50 mm in the last (Gallo et al., 2002; Avila & Papa, 2015).

The fall armyworm is a polyphagous insect and has an adaptation stage in several alternative hosts. In grain and wild sorghum the level is high and intermediate in crops such as soybeans, wheat, brachiaria grass and pigweed. Therefore, the incidence of this pest in corn crops is high (Boregas et al., 2013; Cruz, 2010).

The caterpillar prefers to attack new leaves, scraping them, causing the first symptom known as “scraped leaf”. When the caterpillar occurs inside the ear in formation, direct damage is caused, such as the consumption of dough itself, and indirect damage, facilitating the entry of bacteria and fungi, damaging the quality of the grain and silage. In situations such as high infestation, there may be damage to the collar of plants, stalks and ears. Other factors such as periods of drought, temperatures above 25°C, areas of continuous corn cultivation, off-season corn and late sowing favor the increase in the population of this pest (Avila & Papa, 2015).

For fall armyworm management to be efficient, actions must be carried out throughout the year. Early desiccation is recommended 30 days before sowing, eliminating the presence of green mass (weeds and predecessor crops), which serve as hosts for the pest. In a period close to sowing, a new desiccation must be carried out with the aim of eliminating weeds that were not affected by the first management. In cultural control, practices such as avoiding sowing close to host plants, controlling host plants in the interior and edges, sowing in the opposite direction of the wind and avoiding staggered sowing in the same area or nearby areas, reduce insect pressure (Sarmento, 2002 ).

In biological control, entomopathogens, baculovirus, inoculations of the Trichogramma wasp and the preservation of natural enemies such as earwigs (Doru luteipes), can control efficiently in the field. The most important abiotic factor in controlling fall armyworm is the rainfall regime. When rains are heavy and frequent, the caterpillar is naturally controlled, directly by the rain and indirectly via natural enemies, such as diseases (Sarmento, 2002; Bianco, 1997).

Alternatively, the management of fall armyworm and other lepidopterans can be achieved through the use of Bts corn. These contain proteins that come from bacteria Bacillus thuringienses, which operate throughout the entire crop cycle (Gallo et al., 2002; Santos et al., 2012). However, two legal rules imposed by CTNBio must be respected: one referring to refuge areas, which requires 10% of the area cultivated with corn hybrids not Bt and no more than 800 meters away from the farm Bt, and the other to avoid contamination of neighboring crops that are not Bt, through pollination (Embrapa, 2011).

Hybrids bTS and conventional ones, do not dispense with the use of seed treatments, which must be broad-spectrum, providing efficiency in controlling initial pests. However, the residual effect of seed treatment varies from 15 days to 20 days after emergence, which coincides with the so-called initial pests (Embrapa, 2011).

After the effect of the seed treatment, weekly monitoring of the fall armyworm is necessary. As this pest is not uniformly distributed across areas, both visual monitoring of plant attacks and the use of pheromone traps can be used to justify a control situation. The action level for fall armyworm is reached when 20% of corn plants present level 3 on the Davis scale. In regions with a history of insect pressure, a level of 10% of attacked plants is recommended as a parameter (Grützmacher et al., 2000; Cultivar, 2013; Rosa, 2011; Davis et al., 1992). The control of fall armyworm must be justifiable, that is, through strict monitoring and not through the use of pre-scheduled applications, for example.

If chemical control is required, there are 155 products registered for fall armyworm control., belonging to the chemical groups carbamate, organophosphate, pyrethroid, neonicotinoid, spinosyn, benzoylurea, oxadiazine, diamide, chlorpenapyr and diacylhydrazine and biologicals (Agrofit, 2016). It is preferable to use selective insecticides to preserve natural enemies. The active ingredients must also be rotated to avoid pest resistance (Bianco, 1997).

In the 2014/2015 harvest, 60% of the seeds available on the market were transgenic, resistant to insects of the lepidoptera order and/or resistant to herbicides (Cruz et al., 2014). Of this percentage, there were five transgenic events for the control of Lepidoptera on the market: event MON 89034 (toxin Cry1A.105/Cry2Ab2), VT PRO brand; the event TC 1507 (toxin Bt Cry 1F), Herculex I brand; the MON 810 event (toxin Bt Cry 1Ab), brand YieldGard; the Bt11 event (toxin Bt Cry 1Ab), brand Agrisure TL; and event MIR162 (toxin Bt VIP3Aa20), TL brand Viptera (EMBRAPA, 2014; Cruz et al., 2014).

Two experiments were carried out in the experimental area of ​​the phytotechnics department at UFSM. Visual estimates were made of 10 plants at random in the three central rows of the plot in experiment I and in one row in experiment II, with the aid of the visual scale of damage caused by S. frugiperda, which ranges from 1 to 9 (Davis et al., 1992). Therefore, the objective of this work was to evaluate the presence of the caterpillar Spodoptera frugiperda in corn hybrids using the Davis scale.

Sowing was carried out directly on wheat straw at a density of 6 seeds/m². The experimental design used was completely randomized blocks, with three replications, in both experiments.

The treatments in experiment I consisted of 20 commercial corn hybrids, sown on 19/10/2015. Of these 20, two are (conventional hybrids), 11 (VT PRO), two (Herculex), four (Herculex + YieldGard) and one (YieldGard). Two insecticide applications were carried out: the first in V5, (Thiamethoxan + Lambda-cyhalothrin) and the second in V6 (Lambda-cyhalothrin +Chlorantraniliprole) + (Lufenurom).

In experiment II, the treatments consisted of 10 commercial hybrids and sowing was carried out on 17/12/2015. The following hybrid technologies were used in this experiment: two conventional, two TL Viptera, three VT PRO, one  Herculex, one Herculex + YieldGard and one Agrisure TL. An insecticide application (Acephate) was carried out in V5.

The damage caused by the fall armyworm was evaluated in the phenological stages V4 (4 fully developed leaves) and V8 (8 fully developed leaves), in both experiments. Visual estimates were made of 10 plants at random in the three central rows of the plot in experiment I and in one row in experiment II, with the aid of the visual scale of damage caused by S. frugiperda, which ranges from 0 to 9 (Davis et al., 1992).

In experiment I, in stage V4, the greatest damage was observed in conventional hybrids and those with the Herculex + YieldGard, YieldGard and Herculex technology, with the exception of P 3456 H (Table 1). In the evaluation carried out in the V8 stage, it was possible to observe that the VT PRO technology hybrids were superior to the other technologies (Table 1). It should be noted that in some hybrids, which did not reach grade 3 in V4 and V8, it may not be necessary to apply insecticide, as they did not reach the level of fall armyworm control (Table 1). Hybrids such as BG 7046 and P2530 demonstrated that two insecticide applications, 34 DAS and 40 DAS, were not sufficient to control the caterpillar. In this case, more applications would be necessary, increasing production costs, as the level of control according to Davis et al. (1992), is grade 3.

Moraes et al. (2015), demonstrated that conventional hybrids present greater damage when compared to their transgenic versions in two different locations, Campinas and Mococa and also that different Bts toxins exhibit varying damage due to the attack of S. frugiperda. Furthermore, the demonstration by Moraes et al. (2015) found that the Maximus Viptera event had one of the lowest damage scores and had the highest productivity compared to 9 other hybrids. In this work by Moraes et al. (2015), the lowest scores obtained were for the Herculex and TL Viptera technologies, however the only hybrid with the VT PRO technology did not reach the control level, which is consistent with the majority of hybrids in this work. Martin et al. (2015), when evaluating the damage caused to different corn hybrids by the fall armyworm, they observed that transgenic hybrids were less attacked and that the TL Viptera technology showed less damage in Santa Maria, Rio Grande do Sul.

In experiment II, the evaluation in V4, the cultivars did not reach the necessary control level. However, in V8, the only hybrids with lower damage were Defender VIP and Status VIP. The others showed that an application of insecticide at 34 DAS was not enough to control the caterpillar, and to have avoided this there would have been a control measure before the high values ​​reached at V8. What may explain the high damage in experiment II caused by the fall armyworm is the late sowing, exposing the insect to greater pressure (Avila & Papa, 2015). The low damage of the TL Viptera technology in this experiment is consistent with results from Martin et al. (2015), and Moraes et al. (2015).

Anderson da Costa Rossatto, Guilherme Bergeijer Rosa, Vinícius dos Santos Cunha, Thomas Newton Martin, UFSM 

Article published in issue 205 of Cultivar Grandes Culturas.