insects Article Microbial Control Agents for Fungus Gnats (Diptera: Sciaridae: Lycoriella) Affecting the Production of Oyster Mushrooms, Pleurotus spp. Valerie M. Anderson 1 , Grace F. H. Sward 1, Christopher M. Ranger 2,* , Michael E. Reding 2 and Luis Canas 1 1 Ohio Agricultural Research and Development Center, Department of Entomology, The Ohio State University, Wooster, OH 44691, USA; [email protected] (V.M.A.); [email protected] (G.F.H.S.); [email protected] (L.C.) 2 USDA-Agricultural Research Service, Horticultural Insects Research Lab, 1680 Madison Ave., Wooster, OH 44691, USA; [email protected] * Correspondence: [email protected] Simple Summary: Fungus gnat larvae and adults are challenging insect pests affecting the produc- tion of oyster mushrooms (Pleurotus spp.). The objectives of this study were to develop a container bioassay and assess the bacteria Bacillus thuringiensis var. israelensis (Bti) and entomopathogenic nematode Steinernema feltiae as microbial control agents against fungus gnats. We hypothesized that fungus gnat larvae and the subsequent emergence of adults would be reduced by incorporating Bti and S. feltiae into straw substrate used for producing oyster mushrooms. A series of bioassays were conducted using straw inoculated with Pleurotus columbinus and Pleurotus ostreatus. Fewer fungus gnats emerged from substrate treated with Bti compared to S. feltiae and distilled water. Steinernema Citation: Anderson, V.M.; Sward, feltiae was generally ineffective. Incorporating Bti and S. feltiae into the straw substrate did not appear G.F.H.; Ranger, C.M.; Reding, M.E.; to impede colonization by P. ostreatus. The current study indicates that Bti could be useful as a Canas, L. Microbial Control Agents sustainable pest management technique for producing oyster mushrooms. for Fungus Gnats (Diptera: Sciaridae: Lycoriella) Affecting the Production of Abstract: Infestations of fungus gnats (Diptera: Sciaridae) can reduce the production of oyster Oyster Mushrooms, Pleurotus spp. mushrooms (Pleurotus spp.) grown as food crops within controlled environments. The objectives of Insects 2021, 12, 786. https://doi.org/ this study were to assess the efficacy of Bacillus thuringiensis var. israelensis (Bti) and Steinernema feltiae 10.3390/insects12090786 against fungus gnat larvae. A bioassay was developed, whereby pasteurized straw was inoculated ® ® Academic Editors: Ricardo A. with Pleurotus columbinus and treated with Bti (Gnatrol ), S. feltiae (Nemashield ), or water. Fungus Ramirez and Douglas S. Richmond gnats (Lycoriella sp.) were released into each bioassay container for ovipositing onto the straw, thereby exposing the F1 larvae to treated or untreated substrate. Sticky cards within the containers entrapped Received: 10 August 2021 fungus gnats emerging from the substrate as an indicator of larval survivorship. Following three Accepted: 1 September 2021 bioassays, fewer fungus gnats emerged from straw treated with Bti compared to S. feltiae and the Published: 3 September 2021 water control. Three additional bioassays using Pleurotus ostreatus also demonstrated that fewer fungus gnats emerged from straw treated with Bti compared to S. feltiae and the untreated control. Publisher’s Note: MDPI stays neutral Steinernema feltiae was generally ineffective. Monitoring substrate weight in the bioassay containers with regard to jurisdictional claims in over time indicated that Bti and S. feltiae did not impede colonization by P. ostreatus. Incorporating published maps and institutional affil- Bti into straw substrate is a promising approach for managing fungus gnats infesting Pleurotus spp. iations. Keywords: Pleurotus ostreatus; Pleurotus columbinus; Lycoriella sp.; Bacillus thuringiensis var. israelensis; Steinernema feltiae Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article 1. Introduction distributed under the terms and Oyster mushrooms, Pleurotus spp., are naturally occurring throughout temperate and conditions of the Creative Commons Attribution (CC BY) license (https:// tropical regions of the world [1]. As saprophytic fungi, Pleurotus spp. facilitate wood creativecommons.org/licenses/by/ decomposition, especially dying or dead hardwoods, and are critical for breaking down 4.0/). Insects 2021, 12, 786. https://doi.org/10.3390/insects12090786 https://www.mdpi.com/journal/insects Insects 2021, 12, x FOR PEER REVIEW 2 of 13 1. Introduction Oyster mushrooms, Pleurotus spp., are naturally occurring throughout temperate and tropical regions of the world [1]. As saprophytic fungi, Pleurotus spp. facilitate wood de- composition, especially dying or dead hardwoods, and are critical for breaking down dif- ficult-to-digest components such as cellulose into useable nutrients for the environment [2]. The degradative activity of Pleurotus spp. combined with other fungi and bacteria sub- sequently benefits the soil community and facilitates the formation of fertile soil [3]. Pleurotus spp. are cultivated within controlled environments due to their flavor, me- Insects 2021, 12, 786 dicinal properties, vigorous growth, and undemanding cultivation requirements2 of 13 [4]. The U.S. ranks second to China in the production of edible mushrooms [5]. During 2019–2020, the volume of sales of the mushroom crop in the US totaled 370 million kg of crop valued difficult-to-digest components such as cellulose into useable nutrients for the environ- mentat USD [2]. The1.15 degradative billion [6]. activity Pleurotus of Pleurotus ostreatusspp. combined(Jacq. ex. with fr) otherKummer. fungi and is the bacteria second-most culti- subsequentlyvated edible benefits mushroom the soil communityin the U.S. and and facilitates worldwide the formation after of button fertile soil mushrooms, [3]. Agaricus bisporusPleurotus (Lange)spp. are Imbach. cultivated [5]. within Pleurotus controlled columbinus environments Quel. due is to an their additional flavor, medic- cultivated species inalof oyster properties, mushroom vigorous growth, [7,8]. Oyster and undemanding mushrooms cultivation are excellent requirements sources [4]. The of U.S.protein, vitamins, ranks second to China in the production of edible mushrooms [5]. During 2019–2020, the volumeminerals, of sales and of amino the mushroom acids, along crop in with the US possessing totaled 370 strong million anti-inflammatory kg of crop valued at properties [9– USD11]. 1.15Cultivating billion [6]. oysterPleurotus mushrooms ostreatus (Jacq. is ex. less fr) Kummer.expensive is the than second-most other wood-loving cultivated mushrooms ediblebecause mushroom the growing in the U.S. substrate and worldwide can be after pasteuri buttonzed mushrooms, instead Agaricusof sterilized bisporus [5]. A variety of (Lange)agricultural, Imbach. forest, [5]. Pleurotus and food-processing columbinus Quel. is anwaste additional products cultivated can be species used of as oyster substrates for oys- mushroomter mushrooms [7,8]. Oyster [12]. mushrooms The spent are mushroom excellent sources subs oftrate protein, can vitamins,serve as minerals, feed for livestock and and amino acids, along with possessing strong anti-inflammatory properties [9–11]. Culti- vatingmushroom oyster mushroomsgrowth improves is less expensive the digestibility than other wood-lovingof wheat straw mushrooms [13]. There because is also a compar- theatively growing short substrate time canfrom be pasteurizedinoculation instead to fruiting of sterilized compared [5]. A variety to other of agricultural, edible mushrooms [5]. forest,Like and food-processingother mushroom waste crops products grown can bein usedcontrolled as substrates environments, for oyster mush- producers of oyster roomsmushrooms [12]. The are spent challenged mushroom substrateby dipteran can serve pests, as feed including for livestock sciari andd mushroomflies (Diptera: Sciaridae) growth improves the digestibility of wheat straw [13]. There is also a comparatively short time(Figure from inoculation1), phorid to flies fruiting (Diptera: compared Phoridae), to other edible and mushrooms cecid flies [5]. (Diptera: Cecidomyiidae) [14,15].Like otherYields mushroom of oyster crops mu grownshrooms in controlled are reduced environments, by fly producers larvae feeding of oyster on developing mushroomsmushroom are primordia challenged by[16]. dipteran Larval pests, and including adult fungus sciarid flies gnats (Diptera: can Sciaridae)also spread mushroom (Figuregreen1 ),mold phorid disease, flies (Diptera: Trichoderma Phoridae), spp., and cecidby way flies (Diptera:of carrying Cecidomyiidae) spores on [ 14their,15]. bodies [17–19]. YieldsFurthermore, of oyster mushrooms dead fungus are reduced gnats tend by fly to larvae adhere feeding to onoyster developing mushrooms mushroom as a contaminant primordia [16]. Larval and adult fungus gnats can also spread mushroom green mold disease,increasingTrichoderma processingspp., by time way by ofcarrying producers spores prior on their to shipment. bodies [17– 19Conventional]. Furthermore, insecticides are deadused fungus on a global gnats tendscale to for adhere controlling to oyster dipteran mushrooms pests as aof contaminant oyster mushrooms increasing [15]. However, processingapplications time of by producerssynthetic priorinsecticides to shipment. can Conventional reduce yields insecticides by damaging are used onmushroom a sporo- globalphores, scale lead for controlling to insecticide dipteran resistance pests of oyster development, mushrooms [15 and].