Biological Pest Control from Entomopathogenic Nematodes (EPNs)

The use of pesticides to prevent pest attack is increasing day by day. This has many side effects. Biological pest control plays an important role in reducing these side effects. Biological control has been used in various fields of biology, especially for pesticides and plant growth.

The use of other organisms to control the pests of crops is called bio-control. Currently, the EPN biological control factor is being used to control many important insects worldwide so that many important insect pesticides can be controlled. The use of EPNs to control pesticides is a potential non-chemical approach.

EPN is the only insect parasitic nematode to have an optimal balance of biological control properties. The selection of an EPN to control a particular insect is based on various factors like host chain, host discovery, tolerance of environmental factors, survival and effectiveness of nematodes.

Soil texture, temperature, and host availability are considered important factors in determining their distribution.

What is entomopathogenic nematodes (EPNs)?

EPNs are a group of nematodes (threads or insects), which are soil-dwelling, deadly insect parasites belonging to the phylum (species) nematode of the steinernematid and heterorhabditid families. Entomopathogenic nematodes are soft bodied, non-segmented roundworms that are obligate or sometimes facultative parasites of insects. They are also called endoparasitic nematodes because they live in parasitic form inside the infected pest whereas plant parasitic nematodes do not kill insects but damage plants.

It has proven to be highly effective in biological control of insects living inside and outside the soil, such as insects, butterflies, flies and beetles as well as adults and larvae of beetles, locusts and crickets. The most commonly used biocontrol entomopathogenic nematodes are those used for the control of harmful insects, for example, members of steinernematids and heteroerhabditids. EPNs are associated with bacteria from the transparent family. Heterorhabditis and Steinernema are mutualistically associated with bacteria of the genera Photorhabdus and Xenorhabdus, respectively.

These bacteria are associated with the intestines of nematodes. EPNs differ from other parasitic and plant nematodes, because they kill their hosts (the insect that the nematodes attack) over a relatively short period of time due to their interaction with the bacteria. They make their host fruitless or weak. EPN is specialized in infecting insects only.

The third juvenile stage of EPN is known as Infectious Juvenile (IJ). The infected juvenile is the only free-living stage and is the only stage found outside the insect host. They remain in the soil for several weeks to months in the infectious phase and are capable of infecting many species of insects.

EPNs occur naturally in the soil and detect their hosts in response to carbon dioxide, vibrations and other chemical signals, and they act on chemical stimuli. Infectious juvenile insects enter through the natural gates of the host such as the mouth, anus and thin sections of the host's bark.

After infection, infectious juveniles release the cells of their symbiotic bacteria from their intestines into the blood of the host. The bacterium rapidly folds into the insect's hemolymph and produces toxins, causing the infected insect to die within 24–48 hours of the host.

The body of the insect host killed by steinernematids is cream and white in color, and the insect killed by heteroerhabditids is red and dark brown in color. In the fourth stage, nematodes develop in adolescents and adults.

The progeny pass through four juvenile stages for adult adults. There may be one or more generations within the host carcass and a large number of infectious juveniles are produced, the infective juvenile is released into the environment to infect other host insects and continue their life.

Commercial use of entomopathogenic nematodes (EPN) Steinernema and Heterorhabditis as bioinsecticides.

EPN species	Major pest(s) targeted - as recommended by various commercial companies
Steinernema glaseri	White grubs (scarabs, especially Japanese beetle, Popillia sp.), banana root borers
Steinernema kraussei	Black vine weevil, Otiorhynchus sulcatus
Steinernema carpocapsae	Turfgrass pests- billbugs, cutworms, armyworms, sod webworms, chinch bugs, crane flies. Orchard, ornamental and vegetable pests - banana moths, codling moths, cranberry girdlers, dogwood borers and other clearwing borer species, black vine weevils, peachtree borers, shore flies (Scatella spp.)
Steinernema feltiae	Fungus gnats (Bradysia spp.), shore flies, western flower thrips, leafminers
Steinernema scapterisci	Mole crickets (Scapteriscus spp.)
Steinernema riobrave	Citrus root weevils (Diaprepes spp.), mole crickets
Heterorhabditis bacteriophora	White grubs (scarabs), cutworms, black vine weevils, flea beetles, corn root worms, citrus root weevils
Heterorhabditis megidis	Weevils
Heterorhabditis indica	Fungus gnats, root mealybugs, grubs
Heterorhabditis marelatus	White grubs (scarabs), cutworms, black vine weevils
Heterorhabditis zealandica	Scarab grubs

Benefits of EPN: -

• EPN has the ability to infect a particular insect as well as to infect other insects.

• EPN usually kills insects over a period of 24–48 hours.

• Other chemicals are not required when using EPNs; they can be used with conventional application equipment.

• Use of EPN does not have any side effects on the environment.

• EPN can be increased in number by in vivo and in vitro (solid and liquid culture medium).

• Enough moisture and temperature are required for the EPN to survive and for infectivity.

• EPN has the ability to detect and kill host pests.

• EPNs or the bacteria associated with them have no harmful effects to mammals or plants.

Keep the following points in mind while using EPN: -

• Use EPN to protect soil temperature, soil moisture, sun rays.

• It should be noted that insects match EPN species with the correct species of insecticides to understand their life cycles and functions.

• Matching of the appropriate nematode host-seeking strategy with the insect is necessary because poor host suitability is the most common mistake in the application of EPNs.

• Apart from this, application strategies like field dosage, quantity, irrigation and appropriate application method are very important.

About Authors:

• Mr. Habibul Haque: District Agricultural Officer-Cum-Project Director, ATMA, Dhubri (Assam) - 783324, India.

• Dr. Ashok Kumar Sharma: Principal Scientist & Team Leader, ICAR-DRMR-APART, Bharatpur (Raj.) - 321303, India.

• Mr. Nirmal Chandra Ghose: SDAO (HQ)-Cum-Nodal Officer, ICAR-DRMR-APART, Dhubri (Assam) - 783324, India.

• Dr. Hadi Husain Khan: Research Associate, ICAR-DRMR-APART, Dhubri (Assam) - 783324, India.

• Mr. Pushpendra Singh Sahu: Department of Entomology, SHUATS, Naini, Prayagraj (Allahabad) - 211007 (U.P.), India.