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Abstract
The aim of the current study was to delve into the causes of mortality of Spodoptera littoralis larvae feeding on Metarhizium-colonized plants in the absence of fungal outgrowth on the cadavers as previous studies reported and to elucidate the possible indirect effects of this fungus-colonized diet. The effect was evaluated in experiments conducted using leaf discs of colonized plants and in planta using fungus-colonized whole plants. The mortality rates of larvae fed on Metarhizium-colonized melon leaves were 45.0% and 87.5%, and the average survival times were 6.6 and 3.1 days in experiments performed with discs and in planta, respectively. Notably, these mortality levels were not associated with observed apoptosis mediated by caspases 1, 3-7 and 8; thus, further investigation into the possible immune system reaction of the insect after the ingestion of colonized plants is required. The leaf consumption of S. littoralis larvae fed on melon-colonized leaves was lower than that on control plants in the disc experiments but not in experiments conducted in planta. In this regard, in experiments performed in planta, plant damage increased larval mortality in both fungally challenged and control larvae. There was also a meaningful effect of exposure to Metarhizium-colonized melon leaf discs on S. littoralis fitness, with significant reductions in 39.0% and 22.0% in female fecundity and egg fertility, respectively, detected in females emerging from pupae developing from larvae surviving exposure to colonized plant discs; all larvae died in the in planta experiments. Hence, the present work presents new findings revealing the high potential of endophytic entomopathogenic fungi to improve the outcome of foliar applications against chewing insects in the short, mid- and long term, by the reduction of the reproductive potential of surviving adults and reveals new insights into the development of bioassays with whole plants for more detailed evaluation of the impact of these fungi as endophytes used for plant protection.
Highlights
The use of entomopathogenic fungi (EPFs) in integrated pest management programs as an alternative to less environmentally benign alternatives, such as chemical insecticides, is becoming more frequent [1]
EPFs are contact action microbials that are normally applied via spray application directed at the plant canopy targeting a pest; depending upon the endophytic behavior of the fungal strain, this may lead to temporary colonization of the leaves that improves the outcome of foliar applications of mycoinsecticides against piercing–sucking and chewing insects [2,3]
Leaves treated with the M. brunneum EAMb 09/01-Su strain were endophytically colonized at 48 h, at rates of 64.4% for sprayed leaves used to obtain leaf discs and 95.0% for brushed leaves used in planta
Summary
The use of entomopathogenic fungi (EPFs) in integrated pest management programs as an alternative to less environmentally benign alternatives, such as chemical insecticides, is becoming more frequent [1]. EPFs are contact action microbials that are normally applied via spray application directed at the plant canopy targeting a pest; depending upon the endophytic behavior of the fungal strain, this may lead to temporary colonization of the leaves that improves the outcome of foliar applications of mycoinsecticides against piercing–sucking and chewing insects [2,3]. Strategies such as soil treatments and seed dressing, can take advantage of EPF endophytic behavior, potentially leading to the Insects 2020, 11, 429; doi:10.3390/insects11070429 www.mdpi.com/journal/insects. Its control is becoming more difficult due to resistance and cross-resistance to chemical insecticides and to the bioinsecticide
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