Pathogenicity of indigenous entomopathogenic nematodes from Benin against mango fruit fly (Bactrocera dorsalis) under laboratory conditions
Introduction
Mango (Mangifera indica L., Anacardiaceae) is one of the most important tropical fruits produced in West Africa, a region most favorable for fruit production and export (Vannière et al., 2004, Gerbaud, 2007, Vayssières et al., 2009a). Mango fruit constitutes a very important source of nutrition for rural populations living in northern Benin (Vayssières et al., 2008). In Africa and particularly in Benin, the production of this fruit is confronted with several problems including quality loss due to fruit flies (Tephritidae, Diptera), especially Ceratitis capitata, Ceratitis cosyra and B. dorsalis (Vayssières et al., 2009b). The latter, formerly known as Bactrocera invadens (Schutze et al., 2014), is the most important pest causing serious damage in orchards of mango as well as in other important tropical fruit crops including guava and citrus (Goergen et al., 2011, Vayssières et al., 2009b). Chemical applications have been used as traditional methods to control these fruit flies for many years. For example, Spinosad GF-120 (Spinosad + foodstuff attractant) and Proteus 170 O-TEQ (Thiaclopride + Deltamethrine) showed great performance for control of flies (Vayssières et al., 2009a, N'Depo et al., 2015). However, the environmental side-effects have led to interest in other, environmental friendly, cost effective and locally available control strategies to inhance mango production and export. In this respect, several control methods have recently been developped including the sterile insect technique (Clarke et al., 2011) and the biological control based on the use of weaver ants, Oecophylla smaragdina and Oecophylla longinoda, (Anato et al., 2015, Offenberg et al., 2013, Wargui et al., 2015). Unfortunately, the latter method is associated with some constraints as the ants delay the labor during harvest and are responsible for small black spots left on the fruit (Sinzogan et al., 2008).
EPNs of the genera Steinernema (Panagrolaimomorpha: Steinernematidae) and Heterorhabditis (Rhabditomorpha: Heterorhabditidae) are effective biocontrol agents (Grewal et al., 2005). They have been found in most countries and are successfully used to control many insect pests around the world (Ehlers, 2001). Several strains of Heterorhabditis taysearae, Heterorhabditis indica and Steinernema sp. have been isolated from Benin and all demonstrated a cruiser type insect search strategy (Zadji et al. 2014b). H. taysearae Shamseldean et al., 1996, has been recently considered as a senior synonym of Heterorhabditis sonorensis Stock et al., 2009 by Hunt and Subbotin (2016).
The Infective Juvenile (IJ) represents the only free-living developmental stage of EPNs that occurs naturally in the soil. They are symbiotically associated with bacteria of the family Enterobacteriaceae which belong to the genera Xenorhabdus (Steinernema) or Photorhabdus (Heterorhabditis) (Ciche et al., 2006). IJs of both genera Steinernema and Heterorhabditis can infect the insect larvae via body openings such as anus, mouth or spiracles (Campbell and Lewis, 2002). In addition to these ways of penetrating the insect host, Heterorhabditis species are able to actively enter the hemocoel through the host cuticle by the use of their additional dorsal tooth to perforate the inter-segmental membrane of the cuticle (Bedding and Molyneux, 1982, Griffin et al., 2005). Inside the host they release intestinal bacteria into the insect hemocoel. These bacteria reproduce and produce metabolites that kill the insect within 1–2 days (Dowds and Peters, 2002) and serve at the same time as food source for the nematode. An effective sustainable B. dorsalis management approach could be the use of EPNs to control insect pests at soil-borne stages of the insect life cycle. Indeed, the late larval instar of B. dorsalis leaves the infested fruit and falls on the ground where it burrows in the top 4 cm of the soil prior to pupating after a short dispersal period (Hou et al., 2006). Adult flies emerge from pupae after 1–2 weeks (longer in cool conditions). This offers an opportunity to EPN IJs present in the soil to invade B. dorsalis larvae or pupae even if the exposure time to the larvae is relatively short. Many studies have been conducted on the Mediterranean fruit fly Ceratitis capitata (Gazit et al., 2000, Lindegren and Vail, 1986, Lindegren et al., 1990, Malan and Manrakhan, 2009, Minas et al., 2016, Poinar and Hislop, 1981), the Queensland fruit fly Bactrocera tryoni (Froggatt) (Langford et al., 2014), the cherry fruit fly Rhagoletis cerasi L. (Herz et al., 2006), Bactrocera oleae (Sirjani et al., 2009), Bactrocera cucurbitae, B. dorsalis (Lindegren and Vail, 1986) and the Natal fruit fly Ceratitis rosa (Malan and Manrakhan, 2009) and have demonstrated that the flies were highly susceptible to Steinernema and Heterorhabditis nematodes.
Based on these previous studies and their known biocontrol abilities, EPN of the families Heterorhabditidae and Steinernematidae in association with their symbiotic bacteria Photorhabdus and Xenorhabdus respectively, are considered to be promising biocontrol candidates against B. dorsalis on mango trees in Benin.
Several studies have revealed that indigenous EPNs are well adapted to local environmental conditions and therefore considered as good biological agents to control insect pests (Bedding, 1990, Grewal et al., 1994, Noujeim et al., 2015, Zadji et al., 2014b). To our knowledge, the susceptibility of B. dorsalis to Beninese EPNs has not yet been investigated. The current study is one of a series anticipated for the implementation of cost-effective B. dorsalis management using EPNs in mango orchards in Benin. It aimed to: (i) investigate the occurrence of EPNs in mango orchards in northern Benin, (ii) identify the recovered EPN isolates, (iii) test their pathogenicity against mango fruit fly (B. dorsalis) under laboratory conditions. Specifically, 12 EPN isolates from Benin were screened for their virulence against the third instar larvae of B. dorsalis and the most virulent isolates were selected to investigate the susceptibility of larvae and pupae of B. dorsalis under different abiotic laboratory conditions.
Section snippets
Source of insects
B. dorsalis used in this study were obtained from laboratory rearing initiated from B. dorsalis pupae provided by IITA-Benin (International Institute of Tropical Agriculture-Benin). The original colony of B. dorsalis used at the IITA- Benin institute was established from naturally infested mango fruits collected in Northern Benin. Flies were fed with a mixture of brown sugar and yeast extract at 3:1 proportion (Vayssières et al., 2015). Cages were supplied with water. Ripened papaya fruits were
Nematode occurrence in mango orchards and identification
Two nematode isolates (KorobororouC2 and KorobororouF4) were retrieved from the 70 soil samples taken in mango orchards. This means that 2.86% of soil samples were positive. The two nematode isolates were isolated from two different mango orchards (KorobororouC2: 09°22.356′N/02°41.175′E; KorobororouF4: 09°22.287′N/02°40.233′E) in the same village. They share 100% ITS sequence similarity with each other and with H. taysearae FJ477730 and 99% similarity with H. taysearae EF043443. Molecular
Discussion
In this study, we investigated the susceptibility of B. dorsalis, a serious mango pest in Benin, to indigenous EPNs isolates recovered from soil samples collected in mango orchards and other vegetations in Benin. EPNs are known to be more effective in their natural environment than exotic ones (Bedding, 1990). Therefore, exploring the natural occurrence of EPNs in mango orchards in northern Benin was a first step towards their application in biocontrol. Laboratory investigations to screen
Acknowledgments
We thank Dr. Alexis Onzo for his technical assistance and Dr. Desire Gnanvossou (International Institute of Tropical Agriculture in Benin) for providing scientific knowledge and B. dorsalis live materials (pupae) needed to start our own B. dorsalis rearing at LaPAPP laboratotry (University of Parakou). The first author was supported by a doctoral scholarship from the Special Research Fund BOF-UGent (grant number 01W00713).
References (64)
- et al.
Dangerous liaisons: the symbiosis of entomopathogenic nematodes and bacteria
Biol. Control
(2006) - et al.
Thermal adaptation of entomopathogenic nematodes: niche breadth for infection, establishment, and reproduction
J. Thermal Biol.
(1994) - et al.
Soil type and entomopathogenic nematode persistence
J. Invertebr. Pathol.
(1990) - et al.
Effect of soil temperature, moisture and relative humidity on entomopathogenic nematode persistence
J. Invertebr. Pathol.
(1991) - et al.
Susceptibility of Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), to entomopathogenic nematodes
Biol. Control
(2014) - et al.
Characterization in biological traits of entomopathogenic nematodes isolated from North China
J. Invertebr. Pathol.
(2013) - et al.
Susceptibility of the Mediterranean fruit fly (Ceratitis capitata) and the Natal fruit fly (Ceratitis rosa) to entomopathogenic nematodes
J. Invertebr. Pathol.
(2009) - et al.
Desiccation survival and water contents of entomopathogenic nematodes Steinernema spp. (Rhabditida: Steinernematidae)
J. Invertebr. Pathol.
(1997) - et al.
Evaluation of entomopathogenic nematodes against olive fruit fly, Bactrocera oleae (Diptera: Tephritidae)
Biol. Control
(2009) - et al.
Heterorhabditis sonorensis n. sp. (Nematoda: Heterorhabditidae), a natural pathogen of the seasonal cicada Diceroprocta ornea (Walker) (Homoptera: Cicadidae) in the Sonoran desert
J. Invertebr. Pathol.
(2009)