Nematodes in Bio-controls of Crop Pest
Nematodes are generally fusiform and vermiform shape with a terminal mouth situation on a rounded head and a tail tapered to pointed tip. They are invertebrates. Un-segmented and multi cellular organisms which exhibit bilateral symmetry and bisexuality.
It is the common fact that insect associated nematodes were included as microbial pathogens for pest population suppression. The nematodes species in the Mermithidae, Neotylenchidae and Sterinerema are important. Poinar (1975) recorded 27 families of entomogenous nematodes associated with 19 insect orders. Most of the nematodes that cause injury to their insect hosts are endo-parasitic, occurring in the haemocoel, gut lumen, malpighian tubules, ovaries or other organs. Infection may be either passive such as when eggs of infective juveniles are accidentally eaten by susceptible hosts or active when the infective juveniles penetrate their host somatic cuticles. Nematode activity, in the host may result in sub-lethal injury cause by nutritional depletion or organ disturbance. It may be expressed as retarded growth, reduced activity, lower fecundity, eventually sterility and even the production of inter sexes. On the other hand death generally from the mechanical destruction of host tissue.
Poinar (1971) used a DD-136 strain of Neoaplectana carpoceapsae Weise against 12 insect pests which included from the orders of Lepidoptera Coleoptera and Diptera. The bacterium associated with these nematode names as Achromobacter nematophilus Poinar and Thoma. The infective juveniles of DD-136 are normally ingested by their insect host and once in the gut lumen they pass through the alimentary wall into the haemocoel. There the bacteria are released through the anus which causes a septicemic death to the host in 24-48 hrs. The nematodes feed on the multiplying bacteria and the dead host tissues passing through several generations in which their numbers increase tremendously.
The ensheathed juveniles eventually produced along with their associated specific bacteria, leave the dead depleted host and if they encounter a suitable new host the process is repeated. The complete life cycle takes 5-8 days and in a single were moth larvae (Gelleria mellonella) more than 1,00,000 juveniles may be produced. House et. al. (1965) devised new rearing technique using dog food providing large number of nematodes at low cost. The field test with DD-136 provided variable results against forest and agricultural pest. More than 60% mortality against codling moth, tobacco budworm, Heliothis spp. was recorded. Moisture requirement in the microhabital of this nematodes is important factor to avoid desiccation. They can be applied with wetting agents and the pathogens B. thuringiensis.
Mermithid nematodes parasites of mosquito ate worldwide in distribution. Because of their obligate nature, most are difficult to mass rear economically and hence have been seriously considered as agents for biological insect pest until recent years. They have great potential as they are generally well adopted to their host, fairly host specific and kill their hosts upon emergence. They do not require addition food once they have left their host to mature, mate and deposit their eggs. They have successfully used in field tests. Current work on nematode is on Steinernema spp. and Heterohabditis spp. reared on Gelletia mellonella and used for control of white grubs, cut worms and other soil inhabits insect pests.