SUSCEPTIBILITY OF Elasmopalpus lignosellus

Virulence and concentration of entomopathogenic nematodes (NEPs) in Elasmopalpus lignosellus pupae were evaluated. In the laboratory, the virulence of the isolates Heterorhabditis amazonensis MC01, H. amazonensis JPM3, H. amazonensis GL, Steinernema carpocapsae All and Heterorhabditis sp. Nepet 11 was evaluated and, subsequently, H. amazonensis GL was applied at concentrations of 8; 16; 24 and 32 IJ cm-2. The EPNs were applied to Petri dishes containing ten pupae with five replications. Mortality was assessed every 24 hours for three days. In a greenhouse, H. amazonensis GL was tested at concentrations 24, 25, 26 and 27 IJ cm-2. The IJs were applied in pots containing a 20-cm high ‘BM 3061’ maize plant, besides six pupae with four replications. Knowing that the efficiency of EPNss is directly related to the ability to search and penetrate the host, it was found that H. amazonensis GL is highly virulent to E. lignosellus, presenting an LC50 of 6.49 IJ cm -2 after 48 hours, 5.61 IJ cm-2 in 72 hours and LC90 of 39.70 IJ cm-2 in 48 and 27.73 IJ cm-2 in 72 hours under laboratory conditions. In the soil, pupal mortality was lower and a concentration of 25 IJ cm-2 was responsible for the death of 50% of the population, since environmental variability influences the dynamics of IJ infection and insect defense.

2 nematodes (NEPs) in biological control has already been identified for different insect orders (Woltz et al., 2015;Giometti et al., 2011;Ma et al., 2013). However, it has been shown to occur more efficiently in insects that have at least one stage of their development in the soil, since it is in this environment that nematodes live (Tavares et al., 2007). Two nematode families are considered to have entomopathogenic potential: Steinernematidae and Heterorhabditidae (Nguyen & Hunt, 2007). Infective juveniles (JI) penetrate the host insect through its natural openings (mouth, spiracles and anal and genital pores) and release symbiotic bacteria that produce a wide range of hydrolytic toxins and exoenzymes, which kill the host insect by sepsis, leaving the nematode immersed in the material from which it feeds, enabling its growth and multiplication (Dolinski, 2006;Kaya & Gaugler, 1993).
The use of entomopathogenic organisms for biological control has vast potential, requiring a local search for organisms adapted to climatic conditions and the insect pest (Spiridonov, 2017). Therefore, the objective of this study was to evaluate species of entomopathogenic nematodes (EPNs) with potential to control E. lignosellus pupae and to adjust the application concentration of the most virulent nematode.
The lesser cornstalk borer Elasmopalpus lignosellus Zeller (Lepidoptera: Pyralidae) is an underground pest that occurs from the Southern United States to South America (Viana, 2011). In tropical and subtropical regions, it is considered one of the main pests of maize, attacking the plant at its initial development stage (Chittenden, 1980). Initially, even in the first instars, they feed on new leaves and then penetrate the collar region, making a gallery inside the stem, causing the plant to die (Dupree, 1965). Despite extensive knowledge about the species, management practices are still based on chemical control through seed treatment. However, in areas with low soil moisture, the effectiveness of these products is impaired as, like some herbicides, they require moisture to yield an effective control. These conditions are favorable to the appearance of the caterpillar (Viana, 2011). In addition, chemical control, when misused, can select resistant populations of the insect, causing deleterious effects to the environment, animal health and increased production costs (Dalvi et al., 2011;Zorzetti et al., 2017).
Research has shown interest in using biocontrol agents such as Trichogramma pretiosum Riley (Hymenoptera:
Nevertheless, the impact on lesser cornstalk borer was considered low, since it is sheltered in the plant stem while feeding, or protected by the web shelter that it builds in the soil (Viana, 2011).
The potential use of entomopathogenic pupae were removed for use in the tests.
T. molitor larvae that showed symptoms of infection were washed with water and placed in a dry chamber for 5 days, after which they were removed and placed in White (1927) traps to collect the IJs, according to Molina and López (2001  amazonensis GL isolate and the mortality of E. lignosellus pupae (Figure 1). Probit analysis found that the LC 50 was 6.49 IJ cm -2 in 48 hours and mortality caused by the nematode was verified.  regressions were significant at 0.01 significance by the Chi-square test.
In a similar test, a reduction of more than 90% in the pupal population of Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) was identified, when exposed to H. baujardi LPP7 (Minas, 2008). Under the same conditions, Santos et al. (2011) observed that the mortality of Diabrotica speciosa (Germar) (Coleoptera: Chrysomelidae) pupae increased as the concentration of Heterorhabditis sp. reached rates above 80%. In addition, they found a reduction in population mortality from the concentration of 31 IJ cm -2 .  (Selvan et al., 1993).
Therefore, it can be inferred that the increased concentration of nematodes does not always cause higher host mortality, since there may be a tendency for IJs to be more attracted to insects previously infected by the same organism (Lewis et al., 2002).
In the greenhouse, mortality was statistically different (p-value < 0.001). The concentrations of 25 and 26 IJ cm -2 did not differ between each other and showed a reduction between 33% and 50% in E. lignosellus pupal population ( Table 2).
The maximum reduction in E. lignosellus pupal population caused by H. amazonensis GL in a greenhouse was lower than that identified in the laboratory, indicating that there is an However, field tests must be carried out in order to understand the dynamics and behavior, both of the nematodes and of the insects on the various existing variables.

Conclusions
The isolate H. amazonensis GL showed high virulence to E. lignosellus pupae under the tested conditions. In addition, pupal mortality by entomopathogenic nematodes occurs 48 hours after inoculation.
The concentration of IJ in the laboratory was directly proportional to pupal mortality, with an LC 50 estimate of 6.49 IJ cm -2 in 48 hours and 5.61 IJ cm -2 in 72 hours.
In the greenhouse, a lower pupal mortality was observed, since the variability existing in the environment can influence the dynamics of infection and defense of the nematodes and hosts, respectively. The concentration of 25 IJ cm -2 caused the highest percentage of mortality (50%).