Benefical nematodes, Steinernema riobrave and Steinernema feltiae for plum weevil control
The plum weevil, Conotrachelus nenuphar is a one of the most damaging pests of many fruits including apples, apricots, cherries, nectarines, pears, peaches and plums. Like other insects, the life cycle of the plum weevil consists of four stages including eggs (Non-feeding stage), larvae/grubs (Feeding and Damaging stage), pupae (Non-feeding resting stage) and adults (Reproductive and feeding stage). Adults weevils are small about 5 mm long, brownish- grey in color with four humps on their forewings. The have a typical snout with chewing types of mouthparts. Females of plum weevil lay whitish oval shaped small eggs in the fruits by making small grooves/ cavities under the fruit skin. Eggs hatch into legless larvae/grubs that are creamy white in color with brown heads. After hatching from eggs, larvae start feeding on inside the fruit and while feeding develop through four developmental stages (instars). Mature (fourth stage) larvae then leave the fruits and move into the soil for pupation. In the soil they construct small cells in which they pupate. The plum weevil pupae are generally creamy-white in color. Both adults and larvae of Conotrachelus nenuphar are capable of causing a serious damage to different types of fruits. Adults generally feed on the succulent leaves, petals and small young fruits whereas larvae feed inside the fruits until they become mature. Typical symptoms of damage includes the formation of raised bumps and permanent scars on the surface of the fruits due to feeding and egg laying injuries by adult weevils, respectively. The exit holes on the prematurely dropped fruits indicates symptoms of the internal damage to the fruits caused by feeding by larvae.
Since the plum weevil, Conotrachelus nenuphar is considered as the most destructive pest of many fruits, its control is essential to reduce the losses caused by this pest to the fruit industry. Chemical pesticides can be effective in controlling this insect pest but their use is restricted due to their detrimental effects on the human health and the environment. Therefore, biological control agents including beneficial entomopathogenic fungi, Beauveria bassiana and Metarhizium anisopliae and beneficial nematodes may have a potential to control Plum weevil, Conotrachelus nenuphar. Recently, Shapiro-Ilan et al. (2013) demonstrated that the field application of two beneficial entomopathogenic nematodes including Steinernema riobrave and Steinernema feltiae in the field killed over 97 and 70% soil-dwelling stages of the Plum weevil, Conotrachelus nenuphar, respectively. Of these two beneficial entomopathogenic nematodes, only Steinernema feltiae nematodes are commercially available. For the effective control the plum weevil, Conotrachelus nenuphar, Steinernema feltiae can be applied at the rate of 2300 nematodes per square feet area or 1 billion nematodes/acre or 2.5 billion/hectare using traditional knapsack sprayers.
How beneficial entomopathogenic nematodes kill their insect hosts?
- When the infective juveniles of beneficial entomopathogenic nematodes are applied to the soil surface or mulch, they start searching for their hosts, in this case the plum weevil grubs or pupae.
- Once a weevil grub or pupae has been located, the nematode infective juveniles penetrate into the grub’s body cavity via natural openings such as mouth, anus and spiracles.
- Once in the body cavity, infective juveniles release symbiotic bacteria (Xenorhabdus spp. for Steinernematidae and Photorhabdus spp. for Heterorhabditidae) from their gut in grub’s blood.
- In the blood, multiplying nematode-bacterium complex causes septicemia and kills grubs usually within 48 hours after infection.
- Nematodes feed on multiplying bacteria, mature into adults, reproduce and then emerge as infective juveniles from the cadaver to seek new larvae/grubs/pupae in the soil.
Read following literature on the interaction between beneficial entomipathogenic nematodes and the plum weevil, Conotrachelus nenuphar.
- Alston, D.G., Rangel, D.E.N., Lacey, L.A., Golez, H.G., Kim, J.J. and Roberts, D.W. 2005. Evaluation of novel fungal and nematode isolates for control of Conotrachelus nenuphar (Coleoptera: Curculionidae) larvae. Biological Control 35: 163-171.
- Kim, H.G. and Alston, D.G. 2008. Potential of two entomopathogenic nematodes for suppression of plum Curculio (Conotrachelus nenuphar, Coleoptera: Curculionidae) life stages in northern climates. Environmental Entomology 37: 1272-1279.
- Pereault, R.J., Whalon, M.E. and Alston, D.G. 2009. Field efficacy of entomopathogenic fungi and nematodes targeting caged last-instar plum Curculio (Coleoptera: Curculionidae) in Michigan cherry and apple Orchards. Environmental Entomology. 38: 1126-1134.
- Shapiro-Ilan, D.I., Mizell, R.F. and Campbell, J.F. 2002. Susceptibility of the plum curculio, Conotrachelus nenuphar to entomopathogenic nematodes. Journal of Nematology 34: 246.
- Shapiro-Ilan, D.I., Mizell, R.F., Cottrell, T.E and Horton, D.L. 2004. Measuring field efficacy of Steinernema feltiae and Steinernema riobrave for suppression of plum curculio, Conotrachelus nenuphar larvae. Biological Control 30: 496–503.
- Shapiro-Ilan, D.I., Mizell, R.F., Cottrell, T.E., Horton, D.L. 2008. Control of plum curculio, Conotrachelus nenuphar with entomopathogenic nematodes: Effects of application timing, alternate host plant, and nematode strain. Biological Control. 44: 207-215.
- Shapiro-Ilan, D.I., Wright, S.E.l., Tuttle, A.F., Cooley, D.R. and Leskey, T.C. 2013. Using entomopathogenic nematodes for biological control of plum curculio, Conotrachelus nenuphar: Effects of irrigation and species in apple orchards. Biological Control 67: 123-129.