Impact Of Insect Resistance On on Microbial Control of Insect Pests

Insects are among the earliest and most successful group of animals that exist in a myriad of environment where the potential for infection by microorganisms and parasites is great. Insects have also demonstrated considerable ability to develop resistance to conventional insecticides; and more than 500 species have developed resistance to one or more chemicals (Georghiou, & Lagunes, 1988). As part of a survival strategy, insects have evolved numerous and effective resistance and defense mechanism to most of the conventional chemical insecticides with possession of genes for high levels of oxidase, esterases, glutathione-s-transferases, “insensitive” acetylcholinesterase (AChE), and nerve insensitivity to pyrethroids. Similarly can an insect species susceptible to a pathogen become resistant to more microorganism ?. Host specificity observed with many insect pathogens demonstrates that insect species are naturally resistant to these microorganisms. Indeed, insects that are susceptible to a pathogen can show resistance to various entomopathogens and try to resist infection through morphological, behavioural, developmental (like maturation immunity), physiological, nutritional, biochemical and molecular genetic mechanism etc. In this review different aspects of insects immunity, viz. passive and active defense mechanism against foreign invaders in comparison with vertebrate immunity has been presented. How certain parasites and pathogens like fungus, bacteria ,protozoa, nematodes and virus of insect pests evolved strategies for avoiding both the external barrier as well as the internal immune defense posed by the host insects has also been discussed. Understanding of current knowledge of insect haematology, and molecular basis of the insect biochemical and cellular defense has been stressed for the proper management of pests especially by using various biocontrol agents like parasites and pathogens, including Bt transgenic plants and also for the control of certain insect vectors which carries pathogens that cause human diseases by way of transforming insects themselves.

Insects vs. Vertebrate immunity

Compared to vertebrates, insects do not possess the ability to produce antibodies (immunoglobulins) and do not use immunoglobulin as recognition molecules in the classical sense, against foreign antigen and hence antigenic memory appears to be lacking i.e. (non-memory type). Further, they do not produce alpha/beta interferons (IFN - a /b ) in response to viral infections. Nevertheless, they are capable of “immune” reactions which appears to be predominantly cellular in nature. Several haemolymph induced antibacterial proteins have been reported to be broad-spectrum in their action which are produced in insects in response to bacterial challenger and of shorter duration in nature. This would suggest that analogy to the phenomenon of immunity in vertebrate may be inappropriate, and hence immunity in insects is different from immunity in vertebrates. The defense mechanism in insect is broadly classified into two broad groups. The first one is non-specific immunity which consists of structural and passive barriers like cuticle, gut physio-chemical properties, and peritrophic membrane. The second one is specific immune system involving cellular and humoral immunity which includes the activation of phenoloxoidase cascade, phagocytosis, nodulation (haemocyte aggregation) and encapsulation especially with reference to, bateria ,fungi, protozoa including nematode invaders.

2. NON-SPECIFIC DEFENSE MECHANISMS

2.1 Morphological
An insect resists entomopathogenic nematode (EPN) belonging to Steinernema and Heterorhabditis infection through behavioral, physical, or physiological means, Unlike vertebrates which have extensive exposure of epithelial cells to the external environment, insects have extensive protection of their epithelial tissue. The chitinous cuticle of the insect covers virtually all external surface, even extending through the foregut, hindgut and tracheal tubes, constituting the first line of passive defense in insects. Unlike that of the fore and hindgut, the epithelium of the insect midgut does not have a cuticular lining. However, in many insects there is a membrane called the “peritrophic membrane” which apparently functions to protect cells of the midgut from injury from hard (or) sharp particles of food performing much the same function as mucus in the mammalian alimentary tract. Physical resistance occurs when the nematode cannot penetrate the integument or the cocoon of a host insect. Romanomermis culicivorax has difficulty in penetrating the integument of older mosquito larvae. Dauer juveniles of Steinernema carpocapsae cannot penetrate the silken cocoons of hymenopteran parasitoids. Spiracular openings are portals of entry for EPN, but sieve plates over the spiracles, especially with scarab larvae, may deny nematodes access through this entry point . Avoidence of nematodes due to the presence of thick peritrophic membrane can act as a morphological defenses against EPN.

2.2 Behavioural
Behavioural resistance occurs when the insect actively avoids or repels the nematode. Extremely active mosquito species had a lower prevalence of infection by the mermithid R.. culicivorax than less active ones. Scarab larvae may avoid infection by wiping nematodes away from the mouth. Younger instars of black fly larvae are resistant to infection by Steinernema carpocapsae because the comparatively large nematode is excluded from the insect’s mouth. Aggressive behaviour (grooming with legs, mouth parts, and rasker) of Popillia japonica larvae when nematodes are present on the cuticle thereby removing and/or killing nematodes on the cuticle has also been reported .

2.3 Physiological
Under defense mechanisms, the high gut pH, presence of protease etc; can be found to be detrimental to the infective juveniles of H. bacteriophora . Similarly low gut pH, absence of protease etc; in insect system though not play a key or important resistant mechanism to bacterial pathogen like Bacillus thuringiensis (Bt) and Baculoviruses comprising nucleopolyhedrovirus (NPV) and granuloviruses (GV); have a role to play for their low susceptibility to bacterial and viral pathogens. Grasshoppers/locusts elevate their body temperatures higher than ambient via, habitat selection and/or orientation to solar radiations called ‘basking’ by way intercepting the solar radiation and raising internal thoracic temperature ranging from 380 to 420 c thereby showing ‘behavioural fever’ response. Such a body temperature is predicted to inhibit fungal proliferation, thus giving the host immune system an edge in suppressing the fungus germination and growth which normally lakes place from 25 º to 30 º c, thereby reducing the infection. Thus, thermoregulation by grasshoppers, Melanoplus sanguinipes has been shown to reduce mycosis caused by Beauveria bassiana and Metarhizium anisopliae (Ouedraogo et al.,2003) . Physiological resistance to infection involves the destruction of the nematode by digestive enzymes in the insect’s alimentary tract and the melanization and encapsulation of the nematode within the hemocoel. Melanotic encapsulation of Steinernema carpocapsae has been reported in larvae of several mosquito species. Although the nematode is encapsulated, the majority of the larvae die of septicaemia caused by the bacterium Xenorhabdus nematophilus, which is mutualistically associated with this nematode.

2.4 Biochemical
Eicosanoids is a collective term for all biologically active, oxygenated metabolites of 20: 4n-6 and two other czo poly unsaturated fatty acids. Given the importance of eicosanoid signalling to the insect immune system, interference with eicosanoid metabolism would seem to be a sensible strategy, and thus a potentially important virulence factor for an entomopathogen. Mediation of bacterial clearance has been reported in insects by eicosanoids by way of nodulation on Manduca sexta, Agrotis ipsilon, Pseudaletia unipuncta and Bombyx mori etc.(Jurenka et al., 1997).

2.5 Biotechnological (Molecular)
Very little is known concerning insect defense against virus infection although insect haemocytes can provide cell-mediated immunity to bacterial pathogens through phagocytes and encapsulation. Neither cell mediated or humoral immunity has been demonstrated against virus infection in insects. “Apoptosis” – distinctive type of programmed cell death – a phenomenon evolved as a primitive viral defense in certain vertebrate animals and invertebrates lacking humoral immunity to function as antiviral defense mechanism is gaining importance in cellular defense against viral infections (Narayanan, 1997). However, insect baculoviruses like nuclear polyhedrosis virus, granulosis virus and other DNA viruses, of insects evolved methods apparently to bypass this defense phenomenon of Apoptosis by directly blocking this response with possession of p35 gene (Clem & Miller, 1994). The above findings leads to the future possibility of blocking the apoptosis for increasing virulence of certain baculoviruses as well as for the determination of host range of certain baculovirus and development of robust cells for in vitro multiplication of insect viruses and development of genetically improved parasitoids.

2.6 Genetic
Genetic resistance to bacterial (McGaughey 1985, 1990) and viral (Briese 1986b; ) pathogens has been recorded against pestiferous insects. High levels of resistance to the δ – endotoxin of B. thuringiensis subspecies kurstaki have been recorded for the Indian meal moth, Plodia interpunctella, a pest of stored- grain and cereal products (Mc Gaughey, 1985) . The resistant trait is incompletely autosomal, recessive, and several alleles or genes are believed to be involved and resistance in this insect is linked to an alteration in toxin-membrane binding of the midgut cells (Van Rie et al ;1990).

Resistance of the silkworm to viral diseases such as nuclear (NPV) and cytoplasmic polyhedrosis virus (CPV) and infections flacherie virus (IFV) is controlled by polygenes. The polygenes are supposed to be mainly concerned with defense mechanism of midgut such as antiviral activity of gut juice, characteristic of peritropic membrane, etc. On the other hand non –susceptibility to densonucleosis is controlled by recessive (nsd – 1, nsd – 2) or dominant (nsd –1) major genes. The major gene may cause a deficiency of an enzyme involved in viral multiplication or in the receptor synthesis within the midgut cell. Polygenic resistance can be introgressed into a silkworm variety by selection in a breeding programme of the silkworm variety. A hybrid of two strains usually shows high heterosis in polygenic resistance to viral disease. The breeding procedure for non-susceptible variety to DNV is much easier because the mechanism of non-susceptibility is controlled by a single major gene. The gene can be introduced into the breeding programme or transferred to an existing superior variety by back crossing

3. SPECIFIC DEFENSE MECHANISMS / IMMUNITY

3.1 Cellular immunity
In a cellular defense mechanism, unlike vertebrate which has red blood corpuscles (RBC) and white blood corpuscles (WBC) in a closed circulatory system, insects with open body cavity lack lymphocytes, the major source of vertebrate immunity to virus infection. But they have only free blood cells called haemocytes. Different types of blood cells have important roles in the protection of insects against invading microorganisms. Hence identification and classification of various types of insect blood cells based on the structure and function is important (Gotz & Boman, 1985; Narayanan & Jayaraj, 1973; Narayanan & Subramanian, 1975). Among the six major group of insect haemocytes in recognizing the “self” (Isografts) and non-self (Allografts) , plasmocytes and granulocytes, are the major effector cells and they react to foreign invaders either by phagocytosing like microorganisms or nodulating and encapsulating the objects too large to be individually engulfed,viz. metazoan parasite by way of haemocytes attaching and forming many layers which often become melanotic, thereby causing the death of the parasitoid through starvation and or anoxia mechanism, (Gotz and Boman, 1985). Changes in total haemocytes (THC) during growth and development of healthy insects have been reported by a number of workers (Narayanan1976). Drastic reduction in number of haemocytes during various microbial infection has also been reported by several workers. Infection by B. bassiana results in a gradual suppression of the phagocytic competence of circulating haemocytes and alteration in both total and differential haemocyte counts (DHC) has been reported in the case of fungal (Hung et al., 1979), bacterial (Narayanan & Jayaraj, 1974), viral (Narayanan, 1979), and parasitic infection (Narayanan and Jayaraj, 1973).

3.2 Humoral immunity
Humoral reactions require several hours for their full expression and involve induced synthesis of antibacterial proteins, including ‘cecropins’, ‘attacins’, ‘diptericins’ and ‘defensins’. The detergent properties of their antibacterial proteins disrupt cell membranes of the invading bacteria. Insects also synthesizing lysozome which enzymatically attack bacteria by hydrolyzing their peptidoglycan cell walls. Haemocytic responses feature direct interactions between circulating haemocyte and bacteria, these typically take place immediately after infections.Humoral reaction involve synthesis and release of several antibacterial (immuno) proteins. The antibacterial nature of gut contents (Govindarajan et al., 1975) and partial characterization of haemolymph bacterial proteins (Abraham et al., 1995) has been reported. There are several families of characterized antibacterial proteins like cecropins, attacins, diptericins and defensins (Boman & Steiner, 1981) including recent identification of “Hemolin” (Sun et al., 1990) which belongs to the immunoglobulin superfamily. These insect antibacterial proteins are the best characterized invertebrate antibacterial factors and they have counterparts in mammals . The role of these proteins in non-self recognition as well as acting both prokaryotic and eukaryotic cells including human infectious parasites has been well studied (Kaaya et al., 1987)

4. APPLICATION OF INSECT DEFENSE IN INSECT PEST MANAGEMENT

4.1 PARASITES
Parasites in general, in their habitual host, are able to escape potential lethal defense response posed by the host. Recently it has been shown that females of certain species of endoparasitic wasps belonging to hymenopteran families, Ichneumonidae and Braconidae, produce the particles containing double stranded circular, multipartite DNA virus called “Bracovirus” and “Ichnovirus” respectively and “Polydnavirus”(PDV) in common in the female wasp’s ovarian calyx tissue which enable the parasitoid to initially circumvent host defense These viruses cause host immuno suppression (the equivalent of an insect AIDS – like virus) allowing the parasitoids to mature without invoking a host immune response. The polydna virus triggers apoptosis of host haemocytes, thus causing the host to be immuno suppressed during the initial stages of parasite infection (Strand & Pech,1995). The possibility of using polydna viruses for abrogating the host defense mechanism by “Molecular mimicry” and later by way of changing the haemolymph and the mobility of haemocytes to encapsulate thereby increasing the efficiency of both homologous and heterologous parasites by way of cross protection (Vinson & Stoltz, 1986) thereby making even non-habitual host to become parasitised, has been postulated. Further, future possibility of laboratory breeding of certain parasites for genetic improvement in whatever functions are required for successful parasitism has also been indicated (Strand & Pech, 1995). For eg. In the case of tobacco horn worm Manduca sexta which is semipermissive to Autographa californica multiple nuceopolyhedro virus (AcMNPV), co-infection of Manduca sexta larvae with polydna virus from a braconid parasitoid, Cotesia congregata which produces 33kD PDV- encoded early glycoprotein EPI increases susceptibility to fatal infection by AcMNPV (Washburn et al., 2000).

4.2 ENTOMOPATHOGENS

4.2.1 ENTOMOPATHOGENIC FUNGUS
Insect mycopathogens such as Beauveria bassiana and Metarhizium anisopliae undergo invivo development cycle which includes the following stages (i) adhesion of conidia to the host cuticle, (ii) germ tube formation, (iii) penetration of host cuticle, (iv) vegetative growth within the host and (v) production of externally borne conidio spores. Basic studies on the invivo development of insect mycopathogens like, white muscardine, B. bassiana, green muscardine, M. anisopliae, Nomuraea rileyi, and Verticillium lecanii etc. have addressed the “Determinants” responsible for attachment, germination and cuticular penetration. At present, very little is known about the survival and development of insect mycopathogens within the host insect. Insect myopathogens may overcome the internal defense response by utilizing one or more of the following strategies. Either, the fungal cells developing within the insect may possess an outer coat, which is neutral to circulating haemocytes or they are effectively masked by host proteins or by producing immuno modulating substances which suppress the cellular defense system, thereby the fungal cells may be tolerant to the insects humoral and cellular defense system. In the case of Spodoptera exigua infection by B bassiana results in a gradual suppression of the phagocytic competence of circulating haemocytes especially granular haemocytes and alteration in total and differential haemocyte counts (Hung et al., 1993).

4.2.2 ENTOMOPAHOGENIC BACTERIA
The toxin pathway of Bacillus thuringiensis (Bt) cry protein involves several steps. Upon ingestion by susceptible insect, the insoluble crystalline are solubilized in the gut which has a high alkaline pH of more than 8 and the protoxins are released. These protoxins are then processed by midgut proteases which typically cleave the 130 kD protein into about 500 amino acid from the C-terminus and 28 amino acid from N- terminus into a protease resistant core fragments of active toxins, which passes through the peritrophic membrane and binds to the specific receptors located in the brush border membrane vesicles (BBMV) of the columnar midgut cells of the target tissue. Binding followed by irreversible insertion of the toxins into the apical microvilli membrane of epithelial midgut columnar cell. The interaction of the toxin with those receptors, which has been characterized as an aminopeptidase N (120-180 kD glycoproteins) triggers the formation of ionic channel (pore formation) which disrupt the osmotic equilibrium maintained by the cells by pumping ions into the extracellular medium perforation in the columnar cell apical membrane, renders the cells volume regulation mechanism ineffective. Accordingly the cell swells and ultimately burst by a process known as colloid-osmotic lysis (Gill, 1992). This leads to the disruption of gut integrity and finally the intoxicated insect stop feeding and death of the insect results due to anorexia (cessation of feeding or starvation) or septicaemia (Narayanan, et.al; 1976). Generally pathogenic effect of gram positive, spore forming and crystalliferous bacteria, B. thuringiensis is determined by the activity of the spore to pass through the gut wall, which is lined by the peritrophic membrane. Increased pathogenicity of Bt fed along with boric acid to Spodoptera litura was reported by Govindarajan et al. (1976), demonstrating the destruction of protective layer of peritrophic membrane, thereby facilitating the invasion by the pathogen. Further the role of certain proteolytic immune inhibitor by the gram negative, non-spore forming and non-crystalliferous potential pathogens like Serratia marescens as well as by Bacillus thuringiensis by way of proteolytic digestion of certain insect antibacterial proteins like cecropins and attacins for their successful invasion and infection has been elucidated (Dalhammar & Steiner, 1984)

5. MECHANISMS OF RESISTANCE TO BT

Plodia interpunctella: Reduced binding of Bt toxin to the brush border membrane of the midgut epithelium has been identified as a primary mechanism of resistance in indian meal moth, P. interpunctella with a 50 fold reduction in binding which was correlated with a >100 fold reduction in toxicity to cry IAb. Both midgut pH and altered proteolytic processing were not the major mechanism of resistance (McGaughey, 1985).

Plutella xylostella: As in the case of P. interpunctella reduced binding of Bt toxin has been identified as a primary mechanism of resistance with > 200 fold resistance to cry I Ab in the case of diamondback moth of cabbage, P.xylostella. Both altered proteolytic processing and increased behavioral avoidance in consumption and movement patterns were not a key or important resistanct mechanism in P. xylostella. (Tabashnik et.al;1990)

Heliothis virescens: Somewhat different results have been obtained with a laboratory- selected resistant strain of H. virescens.Brush border membrane vesicle ( BBMV) from the resistant strain showed only a 2-4 fold decrease in binding affinity for cry I Ab and cry I Ac; but an increase of 4-6 fold in the number of binding sites for these two Bt toxins indicating that additional factors must be responsible for the 20 to 70 fold level of resistance exhibited by this strain.

6. RESISTANT MANAGEMENT IN BT TRANSGENICS

Even before the commercial release of transgenic crops, significant levels of Bt resistance have been attained in some insect species, either in the field or in the laboratory. These findings warns us that resistance management cannot be ignored at any stage in the deployment of transgenic Bt products.
Understanding the mechanism of resistance will provide strategies to prevent or delay resistance and hence prolong the usefulness of Bt insecticidal crystal protein’s (ICPs) as environmentally safe insecticides.In recent years, concentration has been raised above the development of resistance in insects to Bt crystal proteins of genetically engineered plants.The following are the various strategies:

6.1 Gene pyramiding
This is based on the presumption that there are an almost unlimited number of different Bt toxins available in nature and that resistance can be managed by using these in various mixture, mosaic, and rotational or sequential system. Recently Chakrabarti et.al.(1998) have reported the synergism of cry 1Ac with cry 1F toxin, by way of lowering EC50 of cry 1Ac toxin to 13 times due to the presence of cry1F, thereby suggesting that the toxins cry1Ac and cry 1F can be expressed together in transgenic crop plants for future effective control of H. armigera and also as for resistance management strategy. Further, Monsanto has developed a two gene product, named Bollgard II, in which the second Bt gene is cry2Ab2 has been incorporated along with cry1Ac, so as to have an additive effect of both the Bt protein with an expanded host range in order to control the fall armyworm Spodoptera frugiperda and beet armyworm S .exigua, S.litura (Narayanan et al., 1975) ( which are resistant to Bt cry 1Ac) along with false American bollworm, Helicoverpa armigera (Mohan & Manjunath, 2002) thereby checking the spread of resistant insects. However caution has to be exercised in future research since already extensive cross resistance among different Bt toxins has been reported in the case of P. xylostella and in the laboratory populations of P. unipuncta.

6.2 Refugia
Facilitating the survival of susceptible insects by way of growing non-transgenic plants along with transgenic plants in a definite ratio, is one of the best theoretical approaches to slow resistance development. This potentially delays the development of insect resistance to Bt crops by providing susceptible insects for mating with resistant insects and thus they mix with genes (McGaughey 1990). This theoretical strategies has got experimental support from the study of (Tabashnik et al., 1994) who has reported rapid reversal of upto 2800- fold resistance to Bt in P. xylostella in the absence of exposure to ICPs which was associated with restoration of ICP binding and increased biotic fitness. Thus the provision of periods/ refugia during which insects are not exposed to Bt is a promising management option.

6.3 Toxin dose acquisition
High dose of Bt which consistently kills heterozygotes along with untreated refuges as a potential means of managing resistance development in transgenic plants, was advocated. This approach maintains constitutive and continuous exposures of Bt toxins in transgenic plants, which is sufficient to kill in the heterozygotes in a population. (McGaughey, 1990).

6.4 Targeted delivery
Since, continuous and constitutive expression of Bt toxic genes may result in selection pressure, tissue specific (use of stem, root, boll, pod or seed) and stage specific promoters along with chemical spray like salicylic acid to induce gene expression at will, aid in delaying the resistant development in insect pests (McGaughey, 1990).

6.5 Second generation toxic genes.
Recently Estruch et al.(1996) characterized the novel insecticidal proteins produced by certain Bt isolates in logarithmic stage of bacteria growth called as vegetative insecticidal proteins (VIP) and Bowen et al., (1998) have characterized four insecticidal toxin from the bacterium Photorhabdus luminescence encoded by toxin complex loci tca, tcb, tcc, and tcd, representing the second generation of insecticidal trans-genes that will complement the novel Bt δ endotoxin in future.

7. INSECT VIRUSES

The study on the insect’s response towards viral invaders has not been extensive and very little has been known regarding insect antiviral immunity. Four virus diseases of the silkworm are known : nuclear polyhedro virus (NPV) cytoplasmic polyhedro virus (CPV), infectious flacherie (IFV), and densonucleovirus (DNV). The NPV infects various tissues and multiplies in the nucleus forming occlusion bodies called polyhedra, which occlude virus particles. CPV infects the midgut epithelium and multiplies in the cytoplasm of columnar cell forming occlusion bodies which occlude virus particles.IFV infects the midgut epithelium and multiplies in the cytoplasm of goblet cell without forming occlusion bodies. The DNV infects the midgut epithelium and multiplies in the nucleus of columnar cell. As early as 1936, the digestive juice of Bombyx mori has been known to have antiviral properties. Recently, advances have been made which demonstrate viral resistance and an insect’s ability to clear viral pathogens. Baculovirus resistance has been observed in inbred insect populations in the laboratory, but whether it will occur extensively to compromise the application of these viruses against field populations remains to be seen (Briese 1986b). Very few field studies have addressed this problem. After a granulosis virus epizootic in Eucosma griseana populations, there was an increase in the LD50 and in the regression slope for this insect the year after the epizootic. Briese (1986b) suggested that the epizootic affected the susceptible population and the residual populations did not show true resistance. Resistance to baculoviruses has been observed in field populations of Spodoptera frugiperda (Fuxa et al., 1988). At the beginning of the season, the larvae are susceptible to the NPV, but later in the season, there is a trend towards reduced susceptibility and increased heterogenicity after exposure to the virus.

7.1 Developmental Resistance
Developmental resistance to NPV infection typically increases with the larval age in the case of H. armigera (Narayanan, 1979). Traditionally, this developmental resistance has been attributed to an increase in biomass (Briese, 1986a). Recently it was shown that active sloughing of the midgut may play a more important role in the developmental resistance than the increased mass (Englehard & Volkman, 1995). In the case of H. zea which is highly refractory host to AcMNPV, Washburn et al, 1996, allowed the icehneumonid parasite, Campoletis sonorensis, before, they orally inoculated them with AcMNPV – hsp 70/lacz. Then they subsequently compared lacz expression in these caterpillars with unparasitised control insects, thereby indicating the cellular immune response. The above study is the first of its kind, from the insecta that implicates the haemocyte encapsulation response in the clearance of viral pathogen and also suggests a possible strategy whereby the baculoviruses can be genetically manipulated to become a more efficacious biopesticide by way of increasing functional host range of baculovirus

7.2 Molecular Mechanism of Resistance
All those insect DNA viruses have evolved methods to by-pass the defense mechanism posed by insects cells either by way of blocking the cellular apoptosis as a part of their invasion strategy or evolved unusual strategy to circumvent apoptosis which are as follows:

7.2.1 i) Occluded baculovirus
Insect baculoviruses like nuclear polyhedrosis virus, granulosis virus and other DNA viruses of insects evolved methods apparently to bypass this defense phenomenon of apoptosis by directly blocking this response with the possession of ‘p35 gene’, thereby monitoring their own survival by suppressing apoptosis of host cells (Clem and Miller, 1994)

7.2.2 ii) Polydna virus
Recently, Strand and Pech (1995) while studying the mechanism underlying the immuno suppression of Pseudoplusia includens (i.e., host) for the parasitization of Microplitis demolitor, found that M. demolitor PDV induced apoptosis in granular cells with characteristic condensation of chromatin, cell surface blebbing and fragmentation of DNA into a 200bp ladder. Thus, the M. demolitor PDV promote their own survival by inducing apoptosis of host immune cells which would otherwise kill the developing M. demolitor egg.

7.2.3 iii) Ascoviruses
These are a new group of viruses that cause a chronic but a fatal disease in lepidopteran larvae. As the disease advances, the host cell divided into membrane bound vesicles (or “sacs”, containing large number of virions), formed by cleavage of infected host cells, accumulate in the haemolymph, imparting it an opaque white colour. The process of vesicle formation and the nature of vesicles themselves strongly resemble apoptosis and apoptotic bodies. These viruses are vectored by parasitoid wasps during oviposition, and the vesicle possibly represents one infectious form of the virus. Thus ascovirus may have developed an unusual strategy for circumventing apoptosis by using a part of their replication pathway (Clem and Miller, 194).

7.2.4 iv) Non-occluded virus
The non-occluded baculoviruses (NOB) do not produce occlusion bodies at any stage in their reproductive cycle. The type species of NOB is the Heliothis Hz-1. Whereas non-occluded baculovirus known as Hz-1 has evolved yet another strategy unlike that of other occluded baculovirus, or ascovirus or polydna virus, to respond to cellular apoptosis. In a typical wild baculovirus infection, polyhedral occlusion bodies are made and are usually visible during the late phase of infection which initiates between 18 and 24h post infection. On the other hand, AcMNPV mutant vAcAnh induces apoptosis around 9-12 h after infection. Whereas in the case of non-occluded Hz-1, it is able to complete its replication rapidly by about 12 h post infection, before apoptosis occurs. Thus, it is able to circumvent the apoptosis efficiently (Clem and Miller, 1994).

7.3 GRANULO VIRUS – ‘Enhancin’
Enhancins which are proteins which are later found to be a metalloproteinase, that are found in certain Granulovirus occlusion bodies, that have the ability to enhance the infection of some nucleopolyhedrviruses by way of degrading insect instestinal mucin (comparable to vertebrate mucin), a major protein constituent of the internal anatomical barrier, viz. peritrophic membrane.

7.4 Entomopoxvirus – Fusolin
Fusolin which are proteins that are found in certain Entomopox viruses (EPV), that have the ability to enhance the NPV infection due to the greater number of NPV virions reaching the microville of midgut susceptible to NPV, since fusolin protein present in the spinoles lead to the disintegration of the peritrophic membrane, which act as a barrier against NPV virions. (Mitsuhashi & Miyamoto, 2003).

8. ENTOMOPATHOGENIC NEMATODES

Inspite of several behavioural defense mechanism exhibited by certain host insects like Papilio japonica by way of aggressiveness, grooming with legs and mouth parts against entomopathogenic nematodes, which has been already discussed; the role of production of certain immune inhibitor by the mutualistic bacteria of Xenorhabdus present in certain Steinernematids and Heterorhabditids of entomopathogenic nematodes in destroying certain antibacterial protein like cecropins and attacins (produced by several host insects ) thereby keeping the cadaver free from putrification by saprophytic microbes has been shown to be effective strategy developed by symbiotic microbes against insect defense mechanism (Gotz & Boman, 1985).

9. CONCLUSION

Some of the important lessons learned from the multitudes of case of insects resistance to organochlorines, organophosphates, carbamates, pyrethroids and insect growth regulators from our past experience includes the following (i) insect populations can and will evolve resistance to novel challenges that are initially devastating to them and (ii) life history characteristics, the available supply of genetic variability and the specifics of the control situation make some species highly prone to evolving resistance and (iii ) there is no guarantee that a safe, effective, inexpensive insecticide can not be misused so as to rapidly induce resistance to it. Development of the genetics of insect pests has lagged far behind that of crop species Arabdopsis thaliana and even farther behind the insect Drosophila melonogastor and nematode Caenorhabdus eleganss etc. Further, the crop gene pool may be under direct human control, but the pest gene pool is not so .It is less widely appreciated, however that the application of molecular genetics to the poor species themselves will be just as important if Bt products are to fulfill their promise.In the case of insect viruses, as it is evident from the discussion above, that cellular defensive strategies apoptosis and insect DNA viral offensive strategies with the possession of p35 gene which inhibit apoptosis ar co-evolved. Hence understanding of which genes play what role is in which tissue of which species is very much important. Thus discussing insect defenses both at cellular and organismal levels will provide information necessary to control or modify host range properties of certain insect virus in future. So, a better understanding of current knowledge of insect haematology and molecular basis of the insect biochemical and cellular defense mechanism pave the way for the proper management of the pests especially by using various biocontrol agents like parasitoids and pathogens after understanding their strategy in bypassing the insect defense. Future possibilities are plenty by way of cloning and expressing some of the antibacterial proteins under insect baculovirus expression system for designing not only for developing effective biocontrol agents but also for the control of certain insect vectors which carries parasites of certain human diseases by way of transforming insect themselves (Kaaya et al;1987).

ACKNOWLEDGEMENTS

The author is grateful to Project Director, Project Directorate of Biological Control, Bangalore for the facilities provided.

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Dr K. Narayanan,
Project Directorate of Biological control,
Hebbal, Bangalore - 560024.