Today, I read a paper published in Journal of Pest Science by Khatri-Chhetri et al. (2011), who tested the efficacy of two newly isolated entomopathogenic nematode species from Nepal against white grub, Holotrichia longipennis.  This white grub is a very serious pest of many crops including black gram, cabbage, chilies, maize, millet, paddy soybean and tomato. Khatri-Chhetri et al. (2011) compared the efficacies of Nepalese strains of entomopathogenic nematodes including Steinernema lamjungense LMT5, S. lamjungense SS4, S. everestense DKP4 with other nematode species including S. abbasi CS1, S. sp. KL1, Heterorhabditis indica CK2 and H. indica CK6) and with an insecticide, chlorpyrifos against second third stages of white grub, Holotrichia longipennis. These researchers reported that when Nepalese entomopathogenic nematodes were applied at the rate of 2.5 billion nematodes per hectare, they were able to reduce over 73% larval populations of white grubs and these results were comparable with efficacy of traditional insecticide, Chlorpyrifos 19 weeks after nematode application. I found that these findings are very exciting because it is always difficult get over 50% mortality of any insect host when entomopathogenic nematodes are applied under field conditions. I think these researchers could have compared 3 and 9 week % corrected mortality data using repeated measure analysis, which would have gave them a better understanding about the nematode efficacy differences between two nematode application time intervals.

Publication:

Khatri-Chhetri, H.B., Timsina, G.P., Manandhar, H.K. and Moens, M. 2011. Potential of Nepalese entomopathogenic nematodes as biocontrol agents against Holotrichia longipennis Blanch. (Coleoptera: Scarabaeidae). Journal of Pest Science 84: 457-469.

Galleria-baiting technique (Bedding and Akhurst, 1975), was used to isolate an entomopathogenic nematode from the soil collected from a citrus orchard in South Africa. Based on molecular characteristics, this new nematode was named as Steinernema citrae that  found to be closely related Stienernema feltiae group.

For detail information read following original paper

Bedding, R.A. and R.J. Akhurst. 1975. A simple technique for detection of insect parasitic rhabditid nematodes in soil. Nematologica. 21: 109-110.

Stokwe, N.F., Malan, A.P., Nguyen, K.B., Knoetze, R. and Tiedt, L. 2011. Steinernema citrae n. sp. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from South Africa. Nematology 13: 569-587.

New entomopathogenic nematode species was found in the soil collected from Atacama Desert in Chile and was named after Atacama Desert as Heterorhabditis atacamensis. I like the way nematode taxonomists (Edgington et al., 2011) used individual morphological characteristics to differentiate this new species from other morphologically similar species of entomopathogenic nematodes. For example, these researchers showed that the H. atacamensis differed from H. marelatus, H. downesi and H. amazonensis based on position of hemizonid (a nematode sensory organ), position of excretory pore and female tail terminus shape, and number and position of genital papillae, respectively.  Using molecular techniques, Edgington et al. (2011) were also able to distinguish H. atacamensis from closely related entomopathogenic nematode species, H. safricana.

 Research Paper

Edgington, S., Buddie, A. G., Moore, D., France, A., Merino, L. and Hunt, D. J. 2011. Heterorhabditis atacamensis n. sp (Nematoda: Heterorhabditidae), a new entomopathogenic nematode from the Atacama Desert, Chile. Journal of Helminthology 85: 381-394.

Based on morphological and molecular characteristics, the nematode isolated from vermicompost using Galleria bait method (Bedding and Akhurst, 1975) has been described as a new entomopathogenic nematode species, Oscheius carolinensis (Ye et al., 2010).  This nematode is also pathogenic to cabbage butterfly (Pieris rapae) and mealworms (Tenebrio molitor).

 Literature

Bedding, R.A. and R.J. Akhurst. 1975. A simple technique for detection of insect parasitic rhabditid nematodes in soil. Nematologica. 21: 109-110.

Ye, W., Torres-Barragan, A. and Cardoza, Y.J. 2010. Oscheius carolinensis n. sp. (Nematoda: Rhabditidae), a potential entomopathogenic nematode from vermicompost. Nematology 12: 121-135.

It is well known fact that the infective juveniles of both Steinernema spp. and Heterorhabditis spp. enter their insect host through natural openings such as mouth, anus and spiracles and eventually reach in the insect body cavity.  As insects do not have a closed circulatory system like animals, their body cavity acts as an open circulatory system, which is filled with the blood that is technically called as hemolymph.  The infective juveniles of Steinernema spp. carry in their gut species specific symbiotic bacteria of the genus, Xenorhabdus whereas the infective juveniles of Heterorhabditis spp. carry in their gut species specific symbiotic bacteria of the genus, Photorhabdus. Once infective juveniles of both Steinernema spp. and Heterorhabditis spp are in the insect body cavity, they release several cells of symbiotic bacteria, Xenorhabdus spp. and Photorhabdus spp., respectively from their gut via anus in the insect blood. Insect blood is conducive for the multiplication of symbiotic bacteria. In the blood, multiplying nematode-bacterium complex causes septicemia and kill their insect host usually within 48 h after infection.  Based on color of insect cadaver, we can easily determine which entomopathogenic nematode-symbiotic bacterium complex was responsible for killing the insect pests. For example, as shown in a picture below, infection by entomopathogenic nematode, Steinernema spp. and symbiotic bacteria, Xenorhabdus spp. complex gives beige color to wax worm cadavers whereas infection by entomopathogenic nematode, Heterorhabditis spp. and symbiotic bacteria Photorhabdus spp. complex gives red color to wax worm cadavers.

To enlarge, click the photo 

Wax worms infected with entomopathogenic nematodes, Steinernema sp and Heterorhabditis sp: Photo by Ganpati Jagdale

Wireworm, Agriotes lineatus cause a tremendous loss to potato yields throughout the world. As biological control agent, entomopathogenic nematodes can serve as a safe alternative to chemical pesticides in managing wireworms and helping to increase potato yields.  It has been shown that the entomopathogenic nematode, Heterorhabditis bacteriophora can cause over 67% mortality of wireworm, Agriotes lineatus within three weeks of application (Ansari et al., 2009).

Literature

Ansari, M.A., Evans, M. and Butt, T.M. 2009. Identification of pathogenic strains of entomopathogenic nematodes and fungi for wireworm control. Crop Protection 28: 269-272.

Sugarcane is grown as an important cash crop in many countries but insect pests like the sugarcane billbug, Sphenophorus levis can cause a tremendous yield loss to this crop. Entomopathogenic nematodes have a great potential to use as a biological control agent against the sugarcane bill bugs. Recently, Giometti et al. (2011) reported that entomopathogenic nematodes including Steinernema brazilense strain IBCB n6 and three strains of Heterorhabditis sp. (IBCB n10, IBCB n24 and IBCB n44) were highly virulent causing over 60% mortality of adults of the sugarcane billbug. Sphenophorus levis.  

Publications:

Giometti, FHC, Leite, LG., Tavares, FM., Schmit, F.S., Batista, A. and Dell’Acqua, R. 2011.  Virulence of entomopathogenic nematodes (Nematoda: Rhabditida) against Sphenophorus levis (Coleoptera: Curculionidae).   Bragantia 70: 81-86.

In the soil environment, infective juveniles of entomopathogenic nematodes (Figure 1.) are always searching for the insect hosts to infect, kill, feed and reproduce.  Once the infective juveniles of both Steinernematid (Steinernema spp.) and Heterorhabditid (Heterorhabditis spp.) nematodes locate any larval, pupal or adult stages of their insect host, they will rush to find any easy entry routes/points to enter into the insect host body.  As shown in Figure 2, the infective juveniles of both Steinernema spp. and Heterorhabditis spp. generally use natural openings such as mouth, anus and spiracles/breathing pores (usually one pair of spiracles per body segment located laterally along the thorax and abdomen of  insects) of their hosts as main points of entry.  However, the infective juveniles of only Heterorhabditis spp. can also enter into host’s body by puncturing the inter-segmental membranes of the cuticle (see Figure 2).  The infective juveniles that enter via mouth and anus will end up in digestive track (gut) whereas those enter through spiracles will reach in tracheal tubes.  However, to kill their host successfully for food and development, the infective juveniles of both Steinernematid and Heterorhabditid nematodes eventually need to penetrate by puncturing digestive track (gut) or tracheal tubules (currently, the process of puncturing is unclear) into insect’s body cavity (an open circulatory system) and release symbiotic bacteria, Xenorhabdus spp. and Photorhabdus spp., respectively from their gut in insect blood generally called hemolymph.  In the blood, multiplying nematode-bacterium complex causes septicemia and kill their insect host usually within 48 h after infection.

To enlarge, click the pictures.

Fig. 1. Infective juveniles of entomopathogenic nematodes- Photo by Ganpati Jagdale

Fig. 2. Points of infection by entomopathogenic nematodes into body of their insect hosts: Photo by Ganpati Jagdale

Insect-parasitic nematodes that belong to both Steinernematidae and Heterorhabditidae families are also called as entomopathogenic nematodes because they cause disease to their insect hosts with the help of mutualistically associated symbiotic bacterial pathogens. The entomopathogenic nematodes that belong to families Steinernematidae (Steinernema spp.) Heterorhabditidae (Heterorhabditis spp.) are symbiotically associated with species specific bacteria, Xenorhabdus spp. and Photorhabdus spp., respectively.  The infective juveniles of entomopathogenic nematodes from both these families carry hundreds of specific bacterium cells in their guts and use them to cause disease and kill their insect host within 48 hours after infection.

Enlarge the picture by clicking on it.

Infective juveniles of entomopathogenic nematodes- Photo by Ganpati Jagdale

Several stored grain/product insect pests like Indian meal moth (Plodia interpunctella), Mediterranean flour moth (Ephestia kuehniella), Sawtoothed grain beetle (Oryzaephilus surinamensis), Mealworms (Tenebrio molitor), Red flour beetle (Tribolium castaneum) and Warehouse beetle (Trogoderma variabile) attack and destroy large quantities of stored grains and products during long-term storage in farm bins, grain processing facilities, warehouses, retail stores, and eventually also on the consumer shelves. The insect pests of stored grain/products have a major economic impact on the food industry due to the costs associated with their management, monitoring, rejection and return of contaminated shipments and failure to meet regulations that required to and pass inspections.  Therefore, there is a need to protect stored food products from attack by insects.  However, stored grain/product pests are generally difficult to control using traditional method as they hide in cracks and crevices, under perforated floors, and inside machinery used for processing of stored-products.  Chemical pesticides are not advisable to use against stored-product pests because of health and environment pollution risks.

The Indian meal moth (Plodia interpunctella): The larval stages infest and feed on different kinds of cereal grains, rice and processed dry foods like pasta, bread and spices.

The Mediterranean flour moth (Ephestia kuehniella): The larval stages mainly feed various types of flour.

The Sawtoothed grain beetle (Oryzaephilus surinamensis): This insect feed on broken seeds and seed germs.

The Mealworm (Tenebrio molitor): Larvae feed on flour and cereals.

The Red flour beetle (Tribolium castaneum): Feed on flour, cereal grains and dried food products like pasta, biscuits etc.

The Warehouse beetle (Trogoderma variabile): Larvae feed on dried cereal grains and food products such as noodles and spaghetti, and dried spices.

Entomopathogenic nematodes:

Entomopathogenic nematodes also called as insect-parasitic nematodes are commercially available and have potential to use as a biological control agent against above stated stored product pests because of their different host finding strategies.

For example, entomopathogenic nematodes, Steinernema carpocapsae use ambush foraging called “sit and wait” strategy to attack highly mobile insects including stored-product pests. After application, infective juveniles of Steinernema carpocapsae will generally remain near or at the surface of the stored-products.  When infective juveniles of Steinernema carpocapsae sense that there is an insect host passing by them, they will attack and infect it by standing on their tails (behavior called ‘nictation’) and jumping on the host.

Ambush foraging entomopathogenic nematode, Steinernema carpocapsae have a capacity to cause over 85% larval mortality of indian meal moths, mediterranean flour moths, mealworms and red flour beetles (Ramos-Rodriguez et al., 2006).

Entomopathogenic nematodes such as Heterorhabditis bacteriophora, Heterorhhabdtits megidis, Steinernema glaseri and Steinernema kraussei are considered as cruiser nematodes because they generally move actively in search of hosts and can easily find and attack their insect hosts that are hiding in deep in the soil or in case of stored-products hiding in cracks and crevices and under perforated floors. Cruiser nematodes never nictate but use carbon dioxide released by insect hosts as cues to attack them. Cruiser entomopathogenic nematodes, Heterorhabditis bacteriophora and Heterorhhabdtits megidis can kill larvae of Indian meal moth (Mbata and Shapiro-IIan, 2005).

Some entomopathogenic nematodes such as Steinernema feltiae and Steinernema 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 and in case of stored-products, pests that hiding in cracks and crevices and under perforated floors or remaining at the surface of the product.

Intermediate foraging entomopathogenic nematode, Steinernema riobrave have a potential to kill over 65% larvae of indian meal moths, mediterranean flour moths, sawtoothed grain beetles, mealworms, red flour beetles and warehouse beetles (Ramos-Rodriguez et al., 2006).

Another intermediate foraging entomopathogenic nematode, Steinernema feltiae can cause over 90% larval mortality of only indian meal moths, mediterranean flour moths, red flour beetles (Ramos-Rodriguez et al., 2006) and over 79% larval mortality of the confused flour beetle,  Tribolium confusum (Athanassiou et al., 2008).

Publications:

Athanassiou, C.G., Palyvos, N.E. and Kakoull-Duarte, T. 2008. Insecticidal effect of Steinernema feltiae (Filipjev) (Nematoda : Steinernematidae) against Tribolium confusum du Val (Coleoptera : Tenebrionidae) and Ephestia kuehniella (Zeller) (Lepidoptera: Pyralidae) in stored wheat.  Journal of Stored Products Research. 44: 52-57.

Athanassiou, C.G., Kavallieratos, N.C., Menti, H. and Karanastasi, E. 2010.  Mortality of four stored product pests in stored wheat when exposed to doses of three entomopathogenic nematodes.   Journal of Economic Entomology 103: 977-984.

Mbata, G.N. and Shapiro-Ilan, D.I. 2005. Laboratory evaluation of virulence of heterorhabditid nematodes to Plodia interpunctella Hübner (Lepidoptera: Pyralidae). Environmental Entomology 34: 676 – 682.

Ramos-Rodriguez, O., Campbell, J.F. and Ramaswamy, S.B.  2006.   Pathogenicity of three species of entomopathogenic nematodes to some major stored-product insect pests. Journal of Stored Products Research 42: 241-252.

Ramos-Rodríguez,O.,Campbell, J. F.,and Ramaswamy, S. 2007. Efficacy of the   entomopathogenic nematodes Steinernema riborave against the stored-product pests Tribolium castaneum and Plodia interpunctella. Biological Control 40:15 -21.

Tradan, S., Vidric, M., and Valic, N. 2006. Activity of four entomopathogenic nematodes against young adult of Sitophilus granarious (Coleptera: Curculionidae) and Oryzophilus surinamensis ( Coleoptera: Silvanidae ) under laboratory condition. Plant Disease and Protection. 113: 168 – 173.

Fayyaz S. and  Javed , S. 2009.  Laboratory Evaluation of Seven Pakistani Strains of Entomopathogenic Nematodes against a Stored Grain Insect Pest, Pulse beetle Callosobruchus chinensis (L.).  Journal of Nematology 41: 255-260.