Specialization character of phytophagous insects is one of the key examples of insect-plant relationship. Host specificity of insect is required for their larval development. Several factors like trade-off, characteristics of host plant, insect’s neural system are responsible to an insect for becoming a specialist. Though specialization character of phytophagy increase the vulnerability towards extinction, still insect tense to be specialist for protecting themselves from natural enemies, getting the microclimate facility and boost up the survival rate of their offspring.
Key words: Phytopagous insect, Plant, Specialist,Generalist,Trade-off,Neural system
For feeding, reproduction and survival purpose organisms used various types of resources. These resources combined termed as ecological ranges of organism. Similarly insect use the plant resources for fulfilling their different requirements such as shelter, food, protection from different predators. However use of host plant range is relatively different in the phytophagous insect species. Entomologist classified the plant feeding insect into two categories such as generalist and specialist according to the mode of host plant use by them.
Generalist insect can be defined those insect which use use wide range of plant species as their host, whereas the specialist insect using a specified range of host plants in their life stages. Again phytophagous insects are differentiated into three categories such as monophagous, oligophagous and polyphagous. The insect species which feed on plants under single genus termed as monophagous.The oligaphous type consumed wide range of plants of different genera but in a single plant family. Whereas a polyphagous insect refers that they are feeding wide range of plant under different plant families (Bernays and Chapman, 1994).
Most of the phytophagous insects are specialized for choosing their host plant. Finding the causes for their specialization is a long discussed issue in insect host plant relationships. Here I discussed some of the reasons why they become specialist though specialist has higher risk of extinction than generalist type of insect.
Offspring's quality versus quantity
The one of the major reasons which creates the difference between generalist and specialist type of insect that is trade off between offspring quality versus quantity. The searching and evaluation of suitable host plant is done mostly by female insect species. The specialist type of insect provides much attention on the analysis of host plant which provides best resource and environment for feeding and survival of their offspring. They put much emphasis on the quality of larvae then the total of number of offspring. Whereas the generalist type insects have different strategy, they searching large number of host plants for increasing the number of their progeny (Janz and Nylin, 1997)
Neural system and plant choice
It is a long doubtful issue that poor neural system is the main reason for host plant specialization character of phytophagous insect. More theoretical studies than practical influence this hypothesis that phytophagous insects become specialist for their less developed neural system. Another predictions recently established that accuracy percentage of decision take by insect is the major cause for making them specialist towards specific host plant (Tosh et al., 2009).But some previous study suggests that poor neural system of phytopgaous insect make them specialist (Tosh et al., 2003).Meanwhile another study which contradict with this previous study, showing that information processing system of specialist insect help them to find out the better quality nettle (Janz and Nylin, 1997).This study also justify that neuron system of specialist phytophagous insect are good enough for discriminating among large number of host plants. However their information system process is greatly depends on olfactory neuron and which plays a significant role in specific host plant selection by specialist (Bernays, 2001).
Taking decision for choosing various host plants or a single host plant is another important factor for comparing the difference between generalist and specialist type of insects. The decision making process of insect is totally controlled by their nervous system. One study suggests that the nervous system of specialist phytophagous insects is quite simple which has less capability to detect multiple host plants. Specialist insect choice their host plant by recognizing the secondary metabolites (one specific host-plant compound) present in the host plant (Tosh et al., 2003). However another study showing that poor neural system is not only responsible for making phytophagous insects into specialist, actually a broad range of host plants contains different stimuli which takes a lot of time from a generalist insect for selecting the right one for it's purpose (Bernays and Funk, 1999).To save the time for searching of suitable host also influence the phytophagous insect to become specialist.
Host plant features
The chemical content of plant is greatly influence the host plant specialization character of insect.Sometimes Oligophagous or polyphagous insect's behavior turned into monophagous character due to presence of specific chemical substance which present in the host plant body.Even insects move to plant species which are under different families where they are getting the required chemical substance. For example, larvae of cabbage white butterfly also choose different plants which glcosinolates rather than Brassicase (Schoonhoven et. al, 2006).Even larval stage of some insect also prefer sulpher contained plants than free of sulpher, because sulpher chemical is beneficial for both larval and adult stage performance of insect (Marazzi and Städler, 2004).
Besides chemical content of plant, latex concentration of plant also play a vital role for specific host plant choice by phytophagous insect.The number of caterpillars is greater in latex free plant species than latex contained plant species (Diniz et al., 1999).
The morphological character of plant specially age of leaf also one of the features which helps the phytophagous insect to take decision about host plant. Polyphagous type under phytophagous insect likes matured, older leaves rather than younger leaves. The reason behind this decision is again chemical content. Because in comparison of older leaves, tender leaves contained huge quantity of toxic secondary chemical which are very harmful for the polyphagous insect. So, insect showing their priority for nesting in the older leaves (Cates, 1980).
Besides these leaf feeding insects, a large number of insects are feeding on different parts of the plant like wood, flower, fruit, and seeds even rooting system. They are specialized on the specific plant parts rather than generalist feeding behavior (Schoonhoven et al., 2006).
Few other reasons
Chemical content, leaf, secondary metabolites of plant has significant influence of host plant selection, but few other reasons like avoid natural enemies and competitors, surrounding micro climate of the host plant. Female insect who's one of the key responsibility of laying eggs, so they choice the host plant which provide maximum number of larva survives. For increasing the survivor ship of larvae they change their habitat area of same plant which provides an enemy free space for their offspring (Wiklund and Friberg, 2008).
From wikipedia, micro climate defined as a tiny atmospheric region which is different from it is surrounding big area ("Microclimate", n.d.).The major variable factors of micro climate are temperature, light intensity and relative humidity. However the change of insect activity is greatly depends on these variable factors. Relative humidity disadvantageously influences the oviposition rate of female insect whereas temperature advantageously influences it. The light intensity of a small area partially affects the population of insects, because insect choice plant species for their mating site by analyzing the intensity of light (Raghu et al., 2004).
Lastly, the list of reasons for becoming a specialist insect is still incomplete. Protection from enemy, utilization of the facilities of microclimate around the host plant and the most significantly the survivorship of progeny seems to be the causes discovered in present days. Though specialization characteristics of insects increase the chance of their extinction due to decreasing number of specific host plant but why they remain specialist is still a big question in the field of evolutionary biology.
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