Endophytic colonization of Vicia faba and Phaseolus vulgaris (Fabaceae) by fungal pathogens and their effects on the life-history parameters of Liriomyza huidobrensis (Diptera: Agromyzidae)

Abstract

Ten fungal isolates belonging to the genera Beauveria (3), Hypocrea (1), Gibberella (1), Metarhizium (2), Trichoderma (1) and Fusarium (2) were evaluated in the laboratory to determine whether they could become endophytic in two pea leafminer (Liriomyza huidobrensis) host plants (Vicia faba and Phaseolus vulgaris), and to assess their possible negative effects on leafminers. Beauveria (ICIPE279), Hypocrea, Gibberella, Fusarium and Trichoderma isolates colonized roots, stems and leaves of both host plant species. Beauveria isolates G1LU3 and S4SU1 colonized roots, stems, and leaves of P. vulgaris but only the root and stem of V. faba. Isolates of Metarhizium failed to colonize the two host plants. The effects of endophytically colonized fungal pathogens on mortality, oviposition, emergence and longevity of L. huidobrensis were investigated after endophytic colonization of V. faba plants. All the fungal isolates that succeeded in colonizing the host plant were pathogenic to L. huidobrensis, causing 100 % mortality within 13.2 ± 0.7–15.0 ± 0.6 d. However, Hypocrea outperformed the other isolates (p < 0.0 001) in reducing longevity of the progeny (11.2 ± 1.0 vs. 17.8 ± 1.4 d in the control), the number of pupae (80.0 ± 6.7 vs. 387.0 ± 21.7 pupae in the control), and adult longevity (3.8 ± 1.0 vs. 9.9 ± 1.8 d in the control). Adult emergence was significantly reduced (p < 0.0 001) in Hypocrea (21.4 %) and Beauveria (38.0 %) treatments compared to the control (82.9 %).

Introduction

Liriomyza spp. (Diptera: Agromyzidae) leafminers are exotic pests of horticultural crops in Africa and have invaded large parts of the continent from the New World (Murphy & LaSalle 1999). The most economically important species include Liriomyza sativae, Liriomyza trifolii and Liriomyza huidobrensis (Chabi-Olaye et al. 2008). Liriomyza species are listed as quarantine pests in overseas markets, especially the European Union (EPPO 2006), and therefore prevent access of Kenyan horticultural products to new market opportunities (KEPHIS 2006). Yield losses of 20–100 % have been reported, depending on crop and location (EPPO 2006). The currently adopted management strategy for their control is largely based on use of synthetic chemical insecticides. In many cases pesticides are not effective due to the development of resistance, elimination of natural enemies and pose health risks due to pesticide residues. Biological control through the use of parasitoids and fungal entomopathogens is being considered as part of an integrated strategy for leafminer management in Kenya (Migiro et al. 2010). Fungal entomopathogens are generally applied in an inundative approach in the crop (Lacey & Goettel 1995). The high cost of fungal production is a limiting factor for the adoption of the technology by growers, in addition to short survival of the inoculum in the environment. Another biocontrol strategy based on fungal entomopathogens consists of disseminating the pathogen among target pest populations by using devices that attract insects to baited stations where they are contaminated with the pathogen and then return to the environment where they can transmit the pathogen to healthy individuals (Vega et al. 2007). Such an approach was evaluated against L. huidobrensis by Migiro et al. (2010). It is now established that many fungal entomopathogens play additional roles such as endophytes in nature (Vega et al. 2009). Fungal endophytes are heterotrophic microorganisms that live inside plants primarily for nutrition, protection and reproduction (Carroll 1988; Azevedo et al. 2000; Backman & Sikora 2008). They have been isolated from many crops including wheat (Larran et al. 2002a), bananas (Pocasangre et al. 2000; Cao et al. 2002), soybeans (Larran et al. 2002b), coffee (Vega et al. 2008), and tomatoes (Larran et al. 2001). Among the fungal entomopathogens, Beauveria bassiana (Hypocreales: Clavicipitaceae) has been reported as an endophyte in maize (Bing & Lewis 1991, 1992a, b; Lomer et al. 1997; Cherry et al. 2004; Orole & Adejumo 2009), sugar beet (Fuller-Schaefer et al. 2005), banana (Akello et al. 2008a), coffee (Vega et al. 2008), tomato (Leckie 2002; Ownley et al. 2004) and cotton (Ownley et al. 2004). Metarhizium anisopliae (Hypocreales: Clavicipitaceae) has been recorded as an endophyte of sugar beet (Fuller-Schaefer et al. 2005) and maize (Akello unpublished data).

Some fungal endophytes protect host plants against plant pathogens (Ownley et al. 2010) and herbivores, including insects (Arnold et al. 2003; Arnold & Lewis 2005; Schulz & Boyle 2005; Rudgers et al. 2007; Vega 2008; Vega et al. 2008). For example, exposure of two aphid species, Rhopalosiphum padi and Metopopophium dirhodum (Hemiptera: Aphididae), and wheat stem sawfly Mayetiola destructor (Diptera: Chloropidae) to wild barley infected with Neotyphodium coenophialum (Hypocreales: Clavicipitaceae) reduced their survival (Clement et al. 1994, 2005). Wheat leaves colonized by either B. bassiana or Aspergillus parasiticus (Eurotiales: Trichocomaceae) reduced the growth rate of Chortoicetes terminifera (Orthoptera: Acrididae) nymphs (Gurulingappa et al. 2010). Endophytic B. bassiana in banana significantly reduced larval survivorship of banana weevil, Cosmopolites sordidus (Coleoptera: Curculionidae), resulting in 42–87 % reduction in plant damage (Akello et al. 2008b). Reduction in feeding and reproduction by Aphis gossypii (Hemiptera: Aphididae) has also been reported on cotton endophytically colonized by either B. bassiana or Lecanicillium lecanii (Hypocreales: Clavicipitaceae) (Gurulingappa et al. 2010). Another possible role for fungal endophytes could include plant growth promotion as well as impact on tritrophic interaction (Harish et al. 2008; Vega et al. 2008; Paparu et al. 2009). Fungal entomopathogens that become established as endophytes can, therefore, play an important role in the regulation of insect populations. The objectives of this study were to determine whether selected fungal isolates are able to endophytically colonize Vicia faba and Phaseolus vulgaris plants, and to assess their negative effects on the life history of L. huidobrensis following endophytic colonization of the host plant.