Keywords: aphid population, environment, limited resources, Voltra model, nSpecies, competition
American Journal of Applied Mathematics and Statistics, 2014 2 (3),
pp 157159.
DOI: 10.12691/ajams2310
Received April 03, 2014; Revised May 14, 2014; Accepted May 14, 2014
Copyright © 2013 Science and Education Publishing. All Rights Reserved.
1. Introduction
Aphids are plant lice small, softbodied sapsucking insect pests in size from 1 to 7 mm with long legs and antennae. Aphids are most destructive insect pests have a wide host range significantly affect various field crops, fruits and vegetables ^{[2]}. Aphids are generally divided in to two subgroups known as greenflies and blackflies and they contain different type of colors such as yellow, pink, and white, etc. Further, more than five hundreds species of aphid population has been found in east part of India. Aphid discharges lethal mucilaginous material, called saliva, smeared over the surface of the leaf preventing the individuals from further sucking and movements causing deaths due to hunger. Some species inject their saliva in to plant roots and slowly damaged the plants and attack a wide range of plant hosts. Some species when moving from one plant to other they spread out plant virus diseases, which has been seen in some soft fruits, such as strawberry and some vegetables such as tomatoes, beets, brinjals, sweet peas, etc. in a similar manner as in wheat ^{[1, 5, 14, 15]}. Some aphid species attack different parts of plants other than leaves. The behavior of beet root aphid penetrate in to the soil and attacks beet roots during most of their life, causing beet plants to faccid and occasionally die if growth rate of aphid population is very high. Aphids are easily driven to death when weather is not according to their survival. Aphids are infertile in extreme temperature because due to higher temperature their symbiotic bacteria are killed on which some aphids are dependent ^{[13, 16]}. The population growth of Soya been, aphin glyeines under varying labels of predator exclusion has been studied ^{[8, 10]}.
Several investigations have been made to study the dynamics of aphid population by using mathematical models ^{[3, 4]}. In experimental observations it is depicted that the total number of individuals in the population of Aphis fabae rapidly increased at the beginning, reaching its maximum on the day and then decreased rapidly as a high consequence of mortality of the aphids ^{[11]}. The first and the simplest law of population growth of a species was given by ^{[9]} as follows,
 (1) 
from which we get
 (2) 
where denotes the population at time , is the population at time and is growth rate of population. The Malthusian growth equation implies that as but later on it has been suggested that crowding affects exist due to excessive demands on a limited food supply, ^{[12]}. He demonstrates that fecundity decreases and even raising deaths due to starvation. It is assumed that due to crowding, the growth rate is reduced by some proportion of the population. Thus, the equation of the population growth of a single species is given by
 (3) 
The population asymptotically approaches to a limiting value (known as carrying capacity) for as .
In these problems, the population of only one biological species has been taken into account. It has been observed that a number of species are competing for a limited resource and the population density of one species may affect the growth rate of the other. In view of the above, we have established a mathematical model for n species competition for aphid population on plant and studied their limiting behavior for limited resources as time approaches to infinity under a particular set of environmental conditions.
2. n Species Competition Model
In a habitat, under consideration, internal competition in a species for limited resources of aphid population is taken into account to describe the phenomenon of nspecies competition model, ^{[6, 7, 17]}. In the proposed model, we assume that is the density of aphid population at time of species. Let denotes the growth rate of pecies in the absence of other species and is the effect of the density of species on the population growth rate of species. Let the area covered by its saliva at time is proportional to the integral then the dynamics of the system is given by the following differential equation,
 (4) 
where, and is positive constant.
To get the solution of non linear differential equation (4) we assume that there are some environmental conditions under which the effect of density of one species on the growth rate of other species is the same as the effect of the density of a species on its own growth rate. From the equation (4) we get,
where .
In the following, we investigate the competitive result for two different types of species (i.e. nd ).
Thus, we can write,
and
Now, assuming, and , we have,
 (5) 
and
 (6) 
Multiplying (5) and (6) by and respectively and then on subtraction we have,
where
 (7) 
Then the integration of the above differential equation leads to
Where being the constant of integration,
 (8) 
It is noted from (8) that,
where and are the value of and at time that is, the initial stage of observation period.
3. Results and Discussion
From equation (8), we note the following two cases,
Case I.
If then as
This implies that , thus we may say that the species become extinct.
Case II.
If then as .This implies that , thus we may say that species becomes extinct.
4. Conclusion
In this paper, we have studied the behavior of the model system of species competition among aphid population. If we identify the effect of species on species and species on species then the competition tensor helps us to predict the behavior of the species of aphid population as the time approaches to infinity. From (7), it can easily be seen that the competition tensor is a skew symmetric tensor (since) and the diagonal elements of the matrix are zero. In view of the cases I and II, we see that if any of the quantity is negative, the species goes to extinction and we will be left with rows. The interaction of aphid species would continue, till it is left with one row.
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