Toxic Bait as an Alternative Tool in the Management of Spodoptera frugiperda in Second Corn Crops

Managing the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith, 1797), has been increasingly difficult owing to the increase in individual resistance to insecticides and genetically modified Bacillus thuringiensis (Bt) plants. This study used the attracting and killing method to control FAW adults in the field by spraying Noctovi with methomyl insecticide. The experiments were conducted in commercial cornfields, non-Bt and Bt crops, over two agricultural years (2018 and 2019) at eight sites distributed in three cities located in the south of Mato Grosso do Sul State, Brazil. The following six insecticide treatments were used: spraying in continuous bands spaced every 100, 50, and 25 m; intermittent spraying every 25 m; control (without insecticide application); and spraying the entire area with insecticide (positive control). Food bait associated with the insecticide molecule was applied to the crop at vegetative stages V1 and V3, and the adult population size and level of leaf damage caused by the caterpillars (assessed via the Davis scale) were evaluated. The application of toxic bait in bands with spacing less than or equal to 50 m significantly reduced the percentage of damage to the plants, with the effect stronger in Bt crops. We suggest that the control of FAW adult populations would be more efficient if the attracting and killing technique was incorporated in integrated pest management programs for second corn crops.

Some pests have evolved resistance to both insecticides and Bt plants, with FAW being the leading example, showing resistance to the Cry1F protein expressed in corn in several countries (Farias et al., 2014;Lu et al., 2010;Santos-Amaya et al., 2015;Storer et al., 2010;Tabashnik et al., 2013) and in Bt plants that express the proteins Cry1A.105 + Cry2Ab2 (Souza et al., 2019).
Thus, the effective implementation of resistance management strategies is essential for ensuring the durability of any control strategy for S. frugiperda. One alternative is to target adults of pest species using semiochemicals, such as pheromones or volatiles from pest host plants (Witzgall et al., 2010). However, one of the limitations to the use of sex pheromones is that they only attract adult males of the target species, whereas food baits composed of vegetable volatiles attract both sexes of various lepidopteran species, which can significantly decrease the adult populations, oviposition, and subsequent larval populations (Del Socorro et al., 2010a;Del Socorro et al., 2010b;Su et al., 2001).
Semiochemicals can be an efficient alternative control method and tool for managing resistance development. Recently, studies conducted under controlled laboratory conditions have demonstrated the possibility of using commercial semiochemicals associated with chemical insecticide molecules for the management of FAW adults (Justiniano & Fernandes, 2020).
Noctovi ® is composed of oleoresins and sugars. The oleoresins release volatiles that attract moths at great distances and the sugars stimulate the moths to feed, also ingesting the lethal insecticide that is associated. Thus, we aimed to evaluate the viability of a new tool for the integrated pest management of second corn crops via the application of the Noctovi ® commercial product with methomyl active insecticide to control FAW adults and reduce caterpillar infestation under real field conditions.

Study Location and Experimental Plots
The experiments were conducted in commercial areas producing second corn crops in three municipalities in the of Mato Grosso do Sul State, Brazil, in eight locations (plots/blocks) in 2018 and 2019 (Table 1). The commercial areas were planted with Bt hybrid VT PRO3 ® , which expresses the proteins Cry1A.105 and Cry2Ab2 for protection against caterpillars, Cry3Bb1 against corn rootworm larva, and CP4-EPSPS that provides tolerance to the glyphosate herbicide, and non-Bt hybrid RR2 ® , which expresses CP4-EPSPS that is used for planting a structured refuge (CIB, 2012).
The corn was sown with 50 cm spacing between the rows, with 3 to 3.2 seeds per meter, resulting in a population between 60,000 and 64,000 plants ha -1 . The control of weeds and non-target insect species in the study areas was undertaken by farmers without distinguishing between treatments and crop technologies.
The experiments were performed in commercial plots of second corn crops, with each replicate using both VT PRO3 ® and RR2 ® technologies. Each experimental plot, located within the producer's commercial area, consisted of 1 ha spaced laterally every 10 m (border). The experimental block was composed of 12 plots, with six Bt and six non-Bt plots ( Figure 1). The equipment speed was 10 km h -1 and the hydraulic circuit pressure was constant between 1.5 and 1.8 bar, resulting in a flow rate between 0.42 and 0.45 L min -1 , which was lower than that indicated in the manufacturer's manual where the tests were performed with water.
The commercial food attractive used, Noctovi ® 43sb, has different characteristics depending on the application modality (applied pure or together with the insecticide without adding water), such as the density and viscosity of the liquid. The mists generated from the applications deposited 1 to 3 drops per 100 cm 2 at spots ranging between 1 and 1.5 cm in diameter.
Treatment 5 (negative control) did not receive any application of insecticide or toxic bait for the management of FAW, whereas in treatment 6, insecticide containing methomyl at a commercial dose of 1.2 L ha -1 diluted in water was applied to the entire area at a rate of 150 L ha -1 . For this, we used model XR110.02 tips with sieves spaced every 0.5 m at a height of 0.5 m above the plants and with a pressure of approximately 3 bar, together with toxic bait application twice at the V1 and V3 stages.
Meteorological conditions, especially rainfall, were monitored throughout the experiment and toxic bait applications because rainfall can drastically reduce the residual persistence of the bait if it occurs soon after application. At least 48 h without rain after application was required to isolate this variable. If this did not occur, all treatments were reapplied.

Data Collection
The S. frugiperda population was monitored weekly by collecting and identifying the adults captured in the sticky bottom of the Delta traps containing sex pheromones. The sticky bottom was replaced during collection to avoid catch failure in the following week.
The damage caused by the attack of caterpillars on corn leaves was quantified according to the damage scale described by Davis, Ng, and Williams (1992), which provides plant damage scores depending on the type, size, and shape of the damage caused to the leaves by FAWs (Table 2). Evaluations occurred at the following five stages (Hanway, 1971;Ransom & Endres, 2014): V1 corresponded to the first leaf; V2 was characterized by two completely expanded leaves, where the insertion ring of the sheath is clearly defined; and V3, V4, and V6 corresponded to three, four, and six completely expanded leaves, respectively. Small, elongated lesions present on whorl leaves and a few mid-sized elongated lesions of 1.3-2.5 cm (1/2-1″) in length present on whorl and/or furl leaves.
Several small-to mid-sized elongated lesions of 1.3-2.5 cm (1/2-1″) in length present on a few whorl and furl leaves.

5
Small, elongated lesions and several mid-sized elongated lesions present on whorl and furl leaves.
* Several large elongated lesions greater than 2.5 cm (1″) in length present on a few whorl and furl leaves and/or a few small-to mid-sized uniform to irregularly shaped holes (basement membrane consumed) eaten from the whorl and/or furl leaves.

6
Small-and mid-sized elongated lesions plus a few large elongated lesions of greater than 2.5 cm (1″) in length present on whorl and/or furl leaves.
* Several large elongated lesions present on several whorl and furl leaves and/or several large uniform to irregularly shaped holes eaten from furl and whorl leaves.

7
Many small-and mid-sized elongated lesions present on whorl leaves plus several large elongated lesions present on furl leaves.
Many elongated lesions of all sizes present on several whorl and furl leaves plus several large uniform to irregularly shaped holes eaten from the whorl and furl leaves.

8
Many small-and mid-sized elongated lesions present on whorl leaves plus many large elongated lesions on the furl leaves.
Many elongated lesions of all sizes present on most whorl and furl leaves plus many mid-to large-sized uniform to irregularly shaped holes eaten from the whorl and furl leaves.

9
Many elongated lesions of all sizes on whorl and furl leaves plus a few uniform to irregularly shaped holes (basement membrane consumed) eaten from the base of the whorl and/or furl leaves.
Whorl and furl leaves almost completely destroyed.
Note. * Scores 5 and 6 adjust for feeding damage caused by migratory mid-instar larvae.    Table 3 of the culture ntly higher tha tments.
eatments nondified corn pla agricultural ye pared to that in reduction, esp damage. Treatments 2, 3, 4, and 6 yielded similar results regarding the percentage of mild damage in the V2 stage and moderate damage in the V4 and V6 stages. Toxic bait applied every 50 and 25 m, continuously and intermittently for V4 and every 25 m for V6, showed similar damage to those in the positive control (treatment 6). Meanwhile, treatment 1 showed similar results to those in the control treatment for Bt corn plots and differed in plots without Bt plants only during the V4 stage, having a higher percentage of mild and moderate damage (Table 3).
Non-Bt Note.
(1) Means followed by the same letters in the columns do not differ by Tukey's test at a significance level of 5%.
(2) C.V. % is the percentage of coefficient variation in the data transformed into √(x + 0.5).

Discussion
Attracting and killing insects by combining bait food and a contact insecticide in a sticky formulation is one method for controlling pests (Poullot et al., 2001). The toxic bait adds a mortality factor, with a synthetic insecticide normally used (Arruda-Gatti & Ventura, 2003). Lepidopterans have been controlled previously in cotton and soybean crops with the application of a mixture of molasses (1 L), water (10 L), and 21.5% methomyl insecticide (30 mL) in 5 L for 5 m of plant line bands at a distance of 50 m (Gallo et al., 2002). Baits with cartap insecticide at doses of 500 and 750 g ha -1 (active ingredient) added to 0.5% sugar were found to effectively control the adult pink bollworm Pectinophora gossypiella population (Papa et al., 2003).
In our study, the infestation of FAW adults was affected by the year, site, and time of sowing, and, consequently, influenced the infestation of caterpillars and damage to crops. However, damage reduction after applying food bait with insecticide sprayed continuously or intermittently in bands spaced 50 and 25 m were similar to each other and to that after application of the methomyl-based insecticide over the entire area.
The application of the Noctovi ® 43sb commercial food bait in combination with the Lannate ® BR (methomyl) insecticide controlled a considerable part of the adult population of S. frugiperda in the treated areas, directly influencing oviposition and, consequently, caterpillar infestation, and reducing damage, as indicated by the mean scores on the Davis scale during the initial stages (V1 to V6) of the second corn crops.
Research under laboratory conditions corroborated the adult FAW mortality caused by toxic bait using the active insecticides methomyl, lambda-cyhalothrin, and spinosad together with the Noctovi ® 43sb commercial food bait, with these mixtures causing 100% mortality in adults up to 5.29 h (Justiniano & Fernandes, 2020).
The attracting and killing technique can be used in integrated pest management programs. However, one of the weaknesses of this method is that its efficiency decreases for high-density pests because there is greater competition for the resource (El-Sayed et al., 2009). Conversely, this technology is highly effective for controlling isolated and low-density populations and can add value to long-term pest management programs (Guerrero et al., 2014).
The combination of toxic baits with crops expressing the proteins Cry1A.105 + Cry2Ab was efficient in managing S. frugiperda caterpillars. In the present study, both technologies showed a significant reduction in damage when using the toxic bait in bands less than 50 m apart. However, the percentage of attacked plants in most stages remained above the recommended level for pest control.
Pest control is recommended in corn plants where 20% and 10% of plants are affected aged up to 30 days and 40-60 days, respectively (Gallo et al., 2002;Rosa & Barcelos, 2012). Regarding the leaf damage index, pest control is recommended where 20% and 10% of plants have Davis scale scores ≥ 3 for non-Bt and Bt, respectively (Bernardi & Omoto, 2018).
The presence of visible damage in areas containing Bt crops and constant reports of insect resistance to Cry1F in Brazil (Farias et al., 2014) indicate that the populations of FAW in Brazil are resistant, even to those plantations containing pyramid resistance genes (Santos-Amaya et al., 2015; Barcelos & Angelini, 2018). On the other hand, insecticides have often shown control failures due to problems in application technology, the habit of these pests that make control difficult and, in some cases, the increase in the frequency of resistant individuals as a result of frequent spraying of insecticides with the same mode of action (Bernardi & Omoto, 2018;Fernandes et al., 2019).
In this way, the management using toxic bait provided a reduction in the percentage of damage caused by the feeding of FAW caterpillars, resembling the standard management of the producer with the application of the insecticide in a total area that also did not keep the damage below the level of pest control threshold. For the high levels of infestation evidenced in these fields, it is necessary to use more management tools to control and maintain the levels of infestation below the control threshold.

Conclusion
The technique of attracting and killing insect pests meets and complies with the general principles of managing resistance to insecticides and Bt plants.
For S. frugiperda moths, the application of this control method using 1 L ha -1 of the toxic bait Noctovi ® 43sb commercial food bait together with the active ingredient methomyl in a dose 60 times lower (20 mL ha -1 ) than the amount sprayed over the entire area, reduced the adult population, and thus significantly decreased the percentage of damage to the crop. Moreover, this effect was stronger in Bt than non-Bt corn crops.
This scenario of medium and high infestation of adults requires the use of other management tools in addition to attracting and killing techniques.