HOST DEFENSE REACTIONS
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A host may ward off a parasitoid either externally before oviposition has occurred, or internally after oviposition, Examples of external defense reactions are given by Cole (1957, 1959) and Hinton (1955). Pupae of certain Lepidoptera (e.g., Aglais urticae and Pararge aegeria) may wiggle vigorously when being attacked by female ichneumonids (Apechthus spp.), and succeed in throwing off the parasitoid. Certain tortrix moth pupae (Cole 1959) avoid parasitism by Apechthus spp. by moving to other parts of their cocoon when it is probed.
The response of the host to attack by the parasitoid can sometimes result in death of the female parasitoids. This has been observed in Goniozus gordhi attacking P. gossypiella (Gordh 1976) and G. emigratus attacking the same host species (Busck 1917). Nickels et al. (1950) reported that Goniozus punctaticeps is often killed by nut casebearer larvae, but rarely is injured by shuckworm larvae. Factors which may contribute to parasitoid injury or death may be the size of other physical features of the ost, the age or physiological condition of the female parasitoid, and the site of attack or ineffectiveness of the venom injected by the female parasitoid.
Internal defense reactions were recognized early by Salt (1941) when he stated, "Far from being a purely passive victim, obliterated without a tract, the host is often able to impress its mark, and a very clear mark at that, upon the insect parasitoid that destroys it. Proof is already available of effects of the host on the size, form, rate of development and behaviour of its parasitoid; evidence can be found of influence on fecundity, longevity and vigour; and it is likely that many other effects still await discovery and description. There can be no doubt that the host may bequeath to its parasitoid an important and sometimes striking legacy of morphological, physiological and behavioristic characters."
Salt continued, "It follows that the host is one of the important factors that must be considered and controlled in any critical, and especially any quantitative study of insect parasitoids. Accounts of behaviour, data on rate of reproduction and longevity, measurements of length, even descriptions of structure--none of these have any absolute validity for the species, none of them can be considered 'biological constants' of a parasitoid, unless the host is known and recorded."
Internal host reactions involve cellular reactions (encapsulation and melanization) and humoral reactions (Griffiths 1960, 1961, 1969)
Encapsulation (or phagocytosis) is the formation of a cyst by host cells around foreign objects. Encapsulation has been reported to occur in epidermal, tracheal, gut, muscle and nervous tissue. Haemolymphic capsules are formed by the haemocytes congregating and differentiating into two layers; and the cells of the inner layer form connective tissue fibers. Opinions differ whether these inner cells form a true syncitium or not, and whether the connective fibers are formed directly from the cytoplasm of the cells or are secreted by them (Bess 1939, Schneider 1950, 1951, Muldrew 1953, Griffiths 1960, Petersen 1962, van den Bosch 1964, Nappi & Streams 1970)..
Melanization in relation to defense reactions involves the deposition of pigment around a parasitoid. Many authors feel that melanization is associated with encapsulation and is essentially a cellular phenomenon. The melanin formed is derived from tyrosine by way of the phenolase reactions. It is thought that the substrate and enzyme are physically separated within certain blood cells normally, but injury causes the reaction to proceed.
Humoral Reactions are poorly understood. Reports suggest that Coccophagus gurneyi female larvae gradually disintegrate in the body fluids of Pseudococcus longispinus without visible reactions. Much the same occurs for Leptomastix dactylopii larvae in Phenacoccus solani and with Monoctonus paladum in the aphid Aulacorthrum circumflexum.
There are two viewpoints concerning the role of encapsulation in defense. One holds that haemocytes play a primary role in causing the death of living eggs and larvae of parasitoids; the other that humoral phenomena cause immunity, and haemocytes merely act as scavengers.
Viewpoint No. 1 is favored because living parasitoids have been found encapsulated. Also, it has been shown that encapsulated Nemeritis developed normally when reinjected into the normal host, Ephestia. Suppression of host reactions in normal hosts revealed that the properties of the surface of Nemeritis eggs and larvae largely determined the extent of their encapsulation in Ephestia.
The immune response varies with the species of host and parasitoid involved. In general different hosts utilize different defense mechanisms against the same parasitoid, and different parasitoids cause similar defense reactions in the same host. Temperature, superparasitism and multiple parasitism also affect immune responses (Salt 1968, van den Bosch 1964).
Encumbered Host Defenses
Although some microorganisms are detrimental to entomophages, Goodwin (1984) noted that entomophagous parasitoids may have symbiotic microorganisms enabling them to successfully attck hosts. For example, Stoltz & Vinson (1979) showed that viruses present in the oviducts of braconids and ichneumonids suppressed the defensive hemocoelic encapsulation process in their hosts.
Exercise 22.1--How may a host defend itself from a natural enemy?
Exercise 22.2--Distinguish between humoral and haemocytic action.
Exercise 22.3--Describe the situation with the alfalfa weevil and its parasitoids.
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