Citrus root weevils

Use insect-parasitic nematodes to control citrus root weevils by Ganpati Jagdale

The citrus root weevil also called as Diaprepes root weevil (Diaprepes abbreviatus) is one of the major insect pests of citrus and many ornamental plants in Florida and California. Several researchers have demonstrated that the application of an insect-parasitic nematode can supress the populations of root weevils in citrus orchards. For example, Steinernema riobrave infective juveniles when applied in citrus orchards or greenhouses can provide 50 to 90% reduction in populations of D. abbreviatus (Bullock et al., 1999; Duncan and McCoy, 1996; Duncan et al., 1996; Shapiro and McCoy, 2000ab).  Applications of S. carpocapsae (All strain), Heterorhabditis bacteriophora (HP-88 strain) or H. bacteriophora (Florida strain) in the citrus grove can also reduce 50-70% adult emergence of D. abbreviatus (Duncan et al., 1996; Schroeder, 1992).  According to Shapiro et al. (1999), S. riobrave, H. bacteriophora and H. indica were highly virulent against younger (50-day-old) than older (100-day-old) D. abbreviatus larvae at 24 or 27 degrees C temperature. Heterorhabditis indica was more virulent than H. bacteriophora in 50-day-old D. abbreviatus larvae at all temperatures. However, H. bacteriophora was more virulent than S. riobrave in 20-day-old larvae at 24 degrees C but it was less virulent than S. riobrave in 50-day-old larvae at 21 degrees C. Please Read following literature for detailed information on interaction between insect-parasitic nematodes and citrus root weevil.

Bullock, R.C., Pelosi, R.R. and Killer, E.E. 1999. Management of citrus root weevils (Coleoptera : Curculionidae) on Florida citrus with soil-applied entomopathogenic nematodes (Nematoda : Rhabditida). Florida Entomologist. 82: 1-7.

Duncan, L.W and McCoy, C.W. 1996 Vertical distribution in soil, persistence, and efficacy against citrus root weevil (Coleoptera: Curculionidae) of two species of entomogenous nematodes (Rhabditida: Steinernematidae; Heterorhabditidae). Environmental Entomology. 25: 174-178.

Duncan, L.W. McCoy, C.W. and Terranova, A.C. 1996. Estimating sample size and persistence of entomogenous nematodes in sandy soils and their efficacy against the larvae of Diaprepes abbreviatus in Florida. Journal of Nematology. 28: 56-67.

Schroeder, W.J. 1992. Entomopathogenic nematodes for control of root weevils of citrus. Florida Entomologist 75: 563-567.

Shapiro, D.I. and McCoy, C.W. 2000a. Susceptibility of Diaprepes abbreviatus (Coleoptera : Curculionidae) larvae to different rates of entomopathogenic nematodes in the greenhouse. Florida Entomologist. 83: 1-9.

Shapiro, D.I. and McCoy, C.W. 2000b. Effects of culture method and formulation on the virulence of Steinernema riobrave (Rhabditida: Steinernematidae) to Diaprepes abbreviatus (Coleoptera: Curculionidae). Journal of Nematology 32: 281-288.

Shapiro, D.I., Cate, J. R., Pena, J., Hunsberger, A. and McCoy, C.W. 1999. Effects of temperature and host age on suppression of Diaprepes abbreviatus (Coleoptera : Curculionidae) by entomopathogenic nematodes. Journal of Economic Entomology. 92: 1086-1092.

How entomopathogenic nematodes find their insect hosts (Foraging Strategies) by Ganpati Jagdale

Infective juveniles of entomopathogenic nematodes use three different strategies to find their insect hosts.1. Ambush foraging: Ambushers such as Steinernema carpocapsae and S. scapterisci have adapted "sit and wait" strategy to attack highly mobile insects (billbugs, sod webworms, cutworms, mole-crickets and armyworms) when they come in contact at the surface of the soil.  These nematodes do not respond to host released cues but infective juveniles of some Steinernema spp can stand on their tails (nictate) and easily infect passing insect hosts by jumping on them.  Since highly mobile insects live in the upper soil or thatch layer, ambushers are generally effective in infecting more insects on the surface than deep in the soil. 2. Cruise foraging: Cruiser nematodes such as Heterorhabditis bacteriophora, H. megidis, Steinernema glaseri and S. kraussei generally move actively in search of hosts and therefore, they are distributed throughout the soil profile and more effective against less mobile hosts such as white grubs and black vine weevils.  Cruisers never nictate but respond to carbon dioxide released by insects as cues. 3. Intermediate foraging: Some nematode species such as Steinernema feltiae and S.riobrave have adapted a strategy in between ambush and cruise strategies called an intermediate strategy to attack both the mobile and sedentary/less mobile insects at the surface or deep in the soil.  Steinernema feltiae is highly effective against fungus gnats and mushroom flies whereas S.riobrave is effective against corn earworms, citrus root weevils and mole crickets.