Assessment of entomopathogenic nematodes and their symbiotic bacteria to control the stink bugs euschistus heros and dichelops melacanthus (Heteroptera: Pentatomidae) in the soybean-corn succession system

Abstract

The Neotropical brown stink bug Euschistus heros and the green-belly stink bug Dichelops melacanthus are the most important pests today for the succession system of soy-corn in Brazil. Dichelops melacanthus attacks the emerging shoot (epicotyl) of corn plants at the seedling stage of their development, remaining and reproducing on the straw over generations. Euschistus heros, on the other hand, feeds directly on the grains and lodge under fallen leaves after the soybean harvest, due to the absence of another crop host. Entomopathogenic nematodes (EPNs) are capable of killing insects due to their ability to search for the host in the soil and to their associations with symbiotic bacteria that cause pathogenesis. This work aimed to assess strains of EPNs and their symbiotic bacteria regarding their potential to control adults of E. heros and D. melacanthus. Entomopathogenic nematodes could be used to kill the stink bugs adults remaining over the straws, while the symbiotic bacteria could be used to kill these insects above ground, and these bacteria are much easier to produce compared to the EPNs. To assess EPNs and their simbiotic bacteria, tests were conducted in the laboratory and under greenhouse conditions. The nematodes S. diaprepesi AM163, S. carpocapsae All and S. carpocapsae IP1 caused 100% mortality of E. heros on the sand substrate, at the rate of 140 IJs/cm² (1000 IJs/insect). The symbiotic bacteria tested topically provided low mortality of E. heros (<40.3%). In a test with S. diaprepesi AM163 applied on the sand substrate at five different rates, after 7 days, mortality of E. heros remained below 50%, except at the high rates of 24 IJs/cm² (50%) and 256 IJs/cm² (80%), respectively. Higher lethality rates also resulted in the increase of the number of IJs produced per insect, up to 88.4 IJs/cm² (5,400 IJs). The nematode S. diaprepesi AM163 applied to E. heros on two substrates (sand and straw) with two-layer thicknesses, at a rate of 88.4 IJs/cm², caused high insect mortality levels (≥69%), regardless of the substrate and its layer thickness. The nematode S. diaprepesi AM163 was twice more virulent to E. heros (69.1% mortality) than to D. melacanthus (28.8%). Dichelops melacanthus was equally resistant to both nematodes S. diaprepesi AM163 and S. carpocapsae IP1. In a greenhouse test, S. diaprepesi AM163 caused again a high mortality level to E. heros (72.5%) at the high rate of 88.4 IJs/cm². Thus, EPNs may turn up an alternative to control E. heros if high rates of production and use become profitable and viable.