First report on the natural occurrence of entomopathogenic fungi in populations of the leafhopper Dalbulus maidis (Hemiptera: Cicadellidae): Pathogen identifications and their incidence in maize crops

Highlights

• The natural occurrence of fungal entomopathogens on Dalbulus maidis is reported

• Two fungal species were found infecting D. maidis adults in two regions of Brazil

• Pathogens were identified as Batkoa sp. and Metarhizium brasiliense

• Coepizootic of Batkoa sp. and M. brasiliense was detected in one of the corn fields

Abstract

The corn leafhopper Dalbulus maidis is one of the most important pests of maize in Latin America. Here we report, for the first time, the natural occurrence of two fungal species infecting the adult stage of this pest. In 2020, insects killed by a pale bluish green fungus in irrigated maize fields located in Northeast Brazil were found attached to the abaxial surface of leaves. Using morphological characters and multigenic phylogeny, it was identified as Metarhizium brasiliense. In the beginning of 2021, the same pathogen was seen on adults in a maize field in the Central-Western region, alongside an entomophthoralean fungus during an epizootic. The latter pathogen was molecularly identified as a species in the genus Batkoa. The number of Batkoa-infected leafhoppers, displaying the typical swollen abdomen and extended wings, reached an average of 1.88 per maize leaf (86.42% of the sampled adults). The incidence of M. brasiliense was higher in plots in the Northeastern region (0.22 and 0.53 adult per leaf) when compared to the Central-Western region (0.04 adult per leaf). The report of D. maidis adults infected by M. brasiliense in agricultural settings located in different geographic regions and over 550 km apart indicates probable widespread occurrence of this pathogen in Brazil. Moreover, this opens the possibility of more applied biological control studies and, perhaps, the development of new tools to manage D. maidis populations.

Introduction

The corn leafhopper Dalbulus maidis (Hemiptera: Cicadellidae) is found in Latin America and some regions of the USA closely associated with maize (Santana Jr. et al., 2019; Summers et al., 2004); however, it may survive in other plants preferably in the same botanical family (Oliveira et al., 2020), and even reproduce in a few unrelated non-hosts infected with mollicute plant pathogens (Purcell, 1988; Sugio et al., 2011). D. maidis is capable of efficiently transmitting the causal agents of maize stunting diseases [Spiroplasma kunkelli (CSS) and the maize bushy stunt phytoplasma (MBSP)] and the Maize rayado fino virus (MRFV) (Nault, 1980; Oliveira et al., 2011). Maize stunting diseases are responsible for large economic losses, which may reduce 70% of grain production (Giménez-Pecci et al., 2002; Massola Jr. et al., 1999). Over the last five years, these diseases have been considered major phytosanitary issues in Brazil (Oliveira et al., 2020). The main control measure for D. maidis populations has been the routine use of systemic insecticides in the early stages of the crop. However, the environmental and safety hazards associated to this practice have increased the interest in the development of alternative control measures (Meneses et al., 2016, Santana Jr. et al., 2019).

A number of invertebrate-pathogenic fungi have been widely commercialised and used to control agricultural pests worldwide (Faria and Wraight, 2007; van Lenteren et al., 2018). Some fungal species within the taxa Metarhizuim and Entomophthorales have known associations with hemipterans in Auchenorrhyncha. For example, the generalist Metarhizium anisopliae has been annually sprayed on enormous acreages of sugarcane and pastures against spittlebugs (Cercopidae) (Toledo et al., 2008a; Li et al., 2010; Mascarin et al., 2019). Other Metarhizium species outside the M. anisopliae complex are adapted to specific hosts (Hu et al., 2014; Zhang et al., 2019), which include hemipteran-specific pathogens. For instance, Metarhizium album, first described by Petch (1931), was found closely associated with leafhoppers (Cicadellidae) in Asia, causing epizootics in field populations (Rombach et al., 1987). More recently, a phylogenetic study showed species of Metarhizium grouping in two distinct clades, one of them with typical Metarhizium-like morphology of the phialides and specific to small planthoppers, members of Delphacidae family (Mongkolsamrit et al., 2020). This clade contains mainly species originally isolated from leafhoppers, including M. album and Metarhizium brasiliense; the latter was originally isolated in Southeastern Brazil in the 1980s from an unidentified Cicadellidae (Kepler et al., 2014).

Entomophthoralean species have also been found naturally infecting and causing epizootics in different small hopper species. In South America, the genera Pandora and Conidiobulus were already reported occurring enzootically on adults of planthoppers associated with rice crops (Toledo et al., 2008b). Infections of spittlebugs by Pandora in Argentina (Foieri et al., 2018) and Batkoa and Furia in Brazil (Leite et al., 2002) were also reported in pastures. Similarly, cicadelids may also succumb to infections caused by this group of fungi. For instance, the occurrence of Zoophthora in populations of leafhopper has been described in several countries (Ben-Ze'ev and Kenneth, 1981; Galaini-Wraight et al., 1991; Mazzoglio et al., 2009).

Despite of the importance of D. maidis in maize production, there is a lack of studies on fungi associated to this pest. Species in the M. anisopliae complex can infect the corn leafhopper under laboratory conditions (Ibarra-Aparicio et al., 2005; Iwanicki et al., 2020), but naturally occurring infections in the field have not been reported. In surveys conducted by our team in 2020 and 2021, a number of dead D. maidis adults were found attached to maize leaves and colonised by two different fungi. Microscopic evaluations revealed that one of the pathogens belongs to the genus Metarhizium, with a candelabrum-like arrangement of phialides and conidia aggregated into prismatic columns. Cadavers colonized by the other fungus had swollen abdomen with fungal outgrowth from intersegmental membranes and extended wings, typical of an entomophthoralean infection. In the present study, we measured the incidence of both fungi in populations of D. maidis adults in maize fields located in two different regions in Brazil, and determined their identity through phylogenetic analyses.