Abstract
The apple maggot, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), is considered a key pest of apples and is native to the eastern United States. The virulence of seven different species of entomopathogenic nematodes (EPN) was assessed against pupae of R. pomonella under laboratory conditions. Nematode species and strains included Steinernema carpocapsae (ALL strain), Steinernema feltiae (SN strain), Steinernema riobrave (355 strain), Steinernema glaseri (VS strain), Heterorhabditis bacteriophora (VS strain), Heterorhabditis indica (HOM1 strain), and Heterorhabditis megidis (UK211 strain). We conducted three bioassays: (i) short-term exposure cup bioassay (7 d), (ii) long-term cup bioassay (30 d), and (iii) pot bioassay (30 d). In the short-term exposure bioassay, all nematode strains (applied at 54 infective juvenile nematodes (IJs) cm−2) significantly reduced (range: 42.9-73.8%) insect survival relative to the control, but no differences were observed among the treatments. For the long-term exposure bioassay, using the same EPN application rate as the short exposure assay, all treatments reduced adult R. pomonella emergence compared with the control. Steinernema riobrave was the most virulent (28.3% survival), and S. glaseri and H. megidis were the least virulent (53.3% survival). In the pot experiment, S. riobrave and S. carpocapsae (applied at 27 IJs cm−2) had the highest virulence (23.3 and 31.7% survival of R. pomonella, respectively), while H. bacteriophora was the least effective (68.33% survival). Our results indicate that S. riobrave, S. carpocapsae, and S. feltiae have substantial potential to attack R. pomonella pupae, and their field application under the tree canopy (prior to adult emergence) in the spring when temperatures are conducive might be a good option for successful IPM of apple maggot fly.
Highlights
Rhagoletis pomonella has one generation per year throughout its range (Dean and Chapman, 1973)
Steinernema riobrave was the most virulent entomopathogenic nematodes (EPN), resulting in the lowest adult emergence (28.3%), which was significantly lower than those treated with S. glaseri and H. megidis (53.3%)
No differences in nematode virulence were observed in the short-term assay, the experiment demonstrated that all EPN species are virulent to the pupae even when the duration of exposure is relatively short
Summary
Rhagoletis pomonella has one generation per year throughout its range (Dean and Chapman, 1973). Adults emerge from puparia beneath infested abandoned or insufficiently managed host trees and immigrate into nearby commercial apple orchards, where they oviposit into fruit flesh (Boller and Prokopy, 1976; Reissig, 1979). Low-maintenance spheres combined with attractive visual cues, a toxicant, and a phagostimulant such as sucrose have provided effective control of R. pomonella in commercial apple orchards (Green and Wright, 2009; Wright et al, 2012; Morrison et al, 2016). All these management strategies target the aboveground adult stage of R. pomonella rather than the soil dwelling stages of R. pomonella. Our focus was on the pupal stage, because among the soil dwelling stages of R. pomonella, the last instar larval stage is very short in tephritids compared with the pupal stage (Prokopy, 1968; Meck et al, 2008) which for R. pomonella remains in the soil for up to eight months depending on location before emerging as adults (Prokopy, 1968)
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