Three beneficial nematodes including Heterorhabditis bacteriophora, Steinernema carpocapsae and Steinernema feltiae have a potential to use as a biological control agents to manage populations of Queensland fruit fly, Bactrocera tryoni, which is one of the most economically important insect pest of many fruit crops.
What are Queensland fruit flies?
Queensland fruit flies are scientifically named as Bactrocera tryon and belong to a family Tephritidae in the Order, Diptera. These fruit flies are mainly native to Australia and known to cause a serious damage to different kinds of fruits including apple (Malus domestica), avocados (Persea americana), apricot (Prunus armeniaca), blackberry (Rubus spp.), cashew (Anacardium occidentale), cherry (Prunus avium), fig (Ficus carica), grapefruit (Citrus paradise), guava (Psidium guajava), lemon (Citrus limon), mango (Mangifera indica), mulberry (Morus spp.), nectarine (Prunus persica), orange (Citrus sinensis;Lloyd et al., 2013), papaya (carica papaya), peach (Prunus persica), pear (Pyrus spp.), passion fruit (Passiflora edulis), persimmon (Diospyros spp.) and plum (Prunus domestica). Queensland fruit flies can also damage vegetables such as cucumbers (Cucumis sativus) and tomatoes (Lycopersicon esculentum). Queensland fruit flies overwinter as adult flies. These adult flies become active early in the summer and start mating. After mating, females lay eggs beneath the fruit skin. Adult flies are about 5-6 mm long, brown colored with yellow markings on their body. The wings fruit flies are transparent with brown markings. Eggs laid by females in the punctures beneath the fruit skin hatch into small cream colored larvae also called maggots. Newly hatched maggots feed on fruit flesh and become mature inside the fruit. The full grown (7-10 mm long) matured maggots then leave the damaged fruit and move in the soil for pupation. Adult flies emerge from pupae and start mating. Adult flies can survive for several weeks by feeding on nectar.
Biological control of Queensland fruit fly with beneficial entomopathogenic nematodes
Beneficial entomopathogenic nematodes mainly from two genera Heterorhabditis and Steinernema have been currently used as environment friendly biological control agents for killing the soil-dwelling stages larvae (caterpillars/ grubs/ maggots), pupae and adults of many insect pests. Keeping this view in mind, researchers from Sydney, Australia tested the efficacy of three commercially available beneficial entomopathogenic nematodes against soil-dwelling stages including matured maggots and pupae of Queensland fruit fly, Bactrocera tryoni under laboratory conditions (Langford et al., 2014). The efficacy of three beneficial entomopathogenic nematode species including Heterorhabditis bacteriophora, Steinernema carpocapsae and Steinernema feltiae was tested using five different rates of nematodes at different moisture levels and temperatures (Langford et al., 2014). According to these researchers, all the three species of beneficial entomopathogenic nematodes were effective in killing maggots but not the pupae of Queensland fruit fly, Bactrocera tryoni. These researchers also demonstrated that cold adapted Steinernema feltiae nematodes were comparatively more effective than other two nematodes in causing mortality of larvae of Queensland fruit fly, Bactrocera tryoni at all concentrations (50-1000 infective juveniles/ square cm), different moisture levels and temperatures (Langford et al., 2014). However, the increased mortality of maggots of fruit flies caused by both Heterorhabditis bacteriophora and Steinernema carpocapsae nematodes was concentration dependent. These results suggest that all the three beneficial entomopathogenic nematodes (see below) can be used as effective biological control agents to manage Queensland fruit flies but their efficacy will depend on the different environmental factors including soil moisture and temperature.
What are Heterorhabditis bacteriophora nematodes?
Beneficial Heterorhabditis bacteriophora nematodes are parasites of insects and belong to a family Heterorhabditidae. These nematodes use “cruise foraging” strategy to find their food (host insects). In this strategy, infective juveniles of Heterorhabditid bacteriophora nematodes generally move actively in search of hosts throughout the soil profile and therefore they are more effective against less mobile hosts such as white grubs and black vine weevils. These nematodes respond to carbon dioxide released by insects as cues to find their hosts. Infective juveniles of this nematode always carry symbiotic bacteria called Photorhabdus luminescens in their guts and use them as a weapon to kill their insect host and as food for their development and reproduction inside the host cadavers. These nematodes enter in the body cavity of their insect hosts through their natural openings such as mouth, anus, spiracles and even through cuticle. After entering into host’s body cavity, nematodes release these bacteria in the insect blood where these bacteria multiply and causes septicemia killing their host usually within 48 h after infection. This nematode is considered as warm temperature adapted nematode because it is effective in killing insect pests in the field when temperature is above 20°C (68°F).
What are Steinernema carpocapsae nematodes?
Beneficial Steinernema carpocapsae nematodes are parasites of insects and belong to a family Steinernematidae. Steinernema carpocapsae nematodes are most effective against many insect pests at temperatures ranging from 22 to 28°C (72 - 82°F). Infective juveniles of Steinernema carpocapsae nematode use an "ambush” foraging strategy to find their insect hosts. In this strategy, the infective juveniles sit-and-wait to attack passing by and highly mobile larvae/ grubs of insects such as armyworms, billbugs, cutworms and sod webworms. Also, infective juveniles of this nematode always carry symbiotic bacteria called Xenorhabdus nematophila in their gut and use them as a weapon to kill their insect hosts and as food for their development and reproduction inside the host cadavers. When the infective juveniles of Steinernema carpocapsae are applied to the soil surface in the fields or thatch layer on golf courses, they can find their victims on or beneath the soil surface, infect and kill them with the help of their symbiotic bacteria usually within 24-48 hours after infection.
What are Steinernema feltiae nematodes?
Beneficial Steinernema feltiae nematodes are also parasites of insects and belong to a family Steinernematidae. Infective juveniles of Steinernema feltiae carry symbiotic bacteria called Xenorhabdus bovienii in their gut as weapon to kill their insect hosts and as food for their development and reproduction inside the host cadavers. This nematode uses intermediate foraging strategy that lies between ambush and cruiser foraging strategies for searching their insect hosts. Like other species of entomopathogenic nematodes, Steinernema feltiae nematodes also enter their insect host’s body cavity through the natural openings such as mouth, anus and spiracles. Once in the body cavity, infective juveniles release their symbiotic bacteria Xenorhabdus bovienii, which multiplies rapidly in the insect blood. In the insect blood, bacteria causes septicemia and kill its host within 48 hours of infection. Beneficial Steinernema feltiae nematodes are considered as cold adapted nematodes because they can be very active and virulent at temperature as low as 10°C (68°F).
Langford, E.A., Nielsen, U.N., Johnson, S.N. and Riegler, M. 2014. Susceptibility of Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), to entomopathogenic nematodes. Biological Control 69: 34-39.
Lloyd, A.C., Hamacek, E.L., Smith, D., Kopittke, R.A. and Gu, H. 2013. Host Susceptibility of Citrus Cultivars to Queensland Fruit Fly (Diptera: Tephritidae). Journal of Economic Entomology 106: 883-890.
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