Mathematical analysis of a model on guava for biological pest control

Lemma 4.1: Guava borers and natural enemies of (1-2) are always non-negative for all time $t \geq 0$ .

Proof: The first differential equation of the model describes the change of guava borer which is given below,

$\frac{d x}{d t}=r x-a x^{2}-b x y$

Taking only linear parts of the above equation, we get

$\frac{d x}{d t} \geq r x$

$\frac{d x}{d t}-r x \geq 0$    (3)

Since the above differential equation is first order differential equation. So we find the integrating factor,

$I . F .=e^{-\int r d t}=e^{-r t}$

Multiplying (3) by the integrating factor $e^{-r t}$ , we get

$e^{-r t} \frac{d x}{d t}-r x e^{-r t} \geq 0$

                or, $\frac{d}{d t}\left(x e^{-r t}\right) \geq 0$   (4)

Integrating (4) both sides with respect to $t$ , we get  $x e^{-r t} \geq c_{1}$   [ $C_{1}$ is an integrating constant] 

                       or,  $x \geq c_{1} e^{r t}$    (5)

For the initial condition $x(0)=x_{0}$ , we obtain the following condition from (5)                

                 $x_{0} \geq c_{1}$    (6)

From (5) and (6) we get,

 $x(t) \geq x(0) e^{r t}, \forall t \geq 0$  and $\mathrm{r} \in \mathrm{R}$

$\because e^{r t}>0$

$\therefore x(t)>0$

Therefore, the guava borer remains non-negative.

Again, the second differential equation of the model describes the change of natural enemies which is given below,

$\frac{d y}{d t}=-s y+c x y$

Taking only linear parts of the above equation, we get

$\frac{d y}{d t} \geq-s y$

$\frac{d y}{d t}+s y \geq 0$    (7)

Since the above differential equation is first order differential equation, so we find the integrating factor,

$I . F .=e^{\int s d t}=e^{s t}$

Multiplying both sides of (7) by $e^{s t}$ , we get,

$e^{s t} \frac{d y}{d t}+e^{s t} s y \geq 0$

              or, $\frac{d}{d t}\left(y e^{s t}\right) \geq 0$    (8)

Integrating both sides (8) with respect to $t$ , we get

$y e^{s t} \geq c_{2}$

         or, $y \geq c_{2} e^{-s t}$     (9)

For the initial condition $y(0)=y_{0}$we obtain the following condition from (9)

$y_{0} \geq c_{2}$

From (9) and (10) we have, $\forall t \geq 0$ and $\mathrm{r} \in \mathrm{R}$ 

Hence, we see that,                                                                             $x(t) \geq 0, y(t) \geq 0$

 $\forall t \geq 0$

Thus all the natural enemies remain non negative.

We can check the stability of different equilibrium points of the guava model and by computing the eigenvalue of Jacobian matrix at equilibrium points. Now the Jacobian matrix of the model (1-2) at the equilibrium point $\left(x^{*}, y^{*}\right)$

$J\left(x^{*}, y^{*}\right)=\left(\begin{array}{cc}{f_{x}\left(x^{*}, y^{*}\right)} & {f_{y}\left(x^{*}, y^{*}\right)} \\ {g_{x}\left(x^{*}, y^{*}\right)} & {g_{y}\left(x^{*}, y^{*}\right)}\end{array}\right)$

or,$J\left(x^{*}, y^{*}\right)=\left(\begin{array}{cc}{r-2 x^{*}-b y^{*}} & {-b x^{*}} \\ {c y^{*}} & {-s+c x^{*}}\end{array}\right)$

At the equilibrium point (0,0) , the Jacobian matrix becomes $J(0,0)=\left(\begin{array}{cc}{r} & {0} \\ {0} & {-s}\end{array}\right)$ .

Now the characteristic equation of Jacobian matrix is

$|J-\lambda I|=0$

or,$\left|\left(\begin{array}{cc}{r} & {0} \\ {0} & {-s}\end{array}\right)-\left(\begin{array}{cc}{\lambda} & {0} \\ {0} & {\lambda}\end{array}\right)\right|=0$

or,$(r-\lambda)(-s-\lambda)=0$

or,$\lambda^{2}+\lambda(s-r)-r s=0$

$\therefore \lambda=r,-S$

Since $r$ is positive and $-S$ is negative. So the equilibrium point (0,0) is unstable.

Again, we check the stability of the equilibrium point $\left(\frac{s}{c}, \frac{r c-a s}{b c}\right)$.

The Jacobian matrix becomes,

$J\left(\frac{s}{c}, \frac{r c-a s}{b c}\right)=\left(\begin{array}{cc}{r-2 a \frac{s}{c}-b \frac{r c-a s}{b c}} & {-b \frac{s}{c}} \\ {c \frac{r c-a s}{b c}} & {-s+c \frac{s}{c}}\end{array}\right)$

$\therefore J\left(\frac{s}{c}, \frac{r c-a s}{b c}\right)=\left(\begin{array}{cc}{\frac{-a s}{c}} & {\frac{-b s}{c}} \\ {\frac{r c-a s}{b}} & {0}\end{array}\right)$

Now the characteristic equation of the system is, $|J-\lambda I|=0$

or $| \| \begin{array}{cc}{\frac{-a s}{c}} & {\frac{-b s}{c}} \\ {\frac{r c-a s}{b}} & {0}\end{array} )-\left(\begin{array}{ll}{\lambda} & {0} \\ {0} & {\lambda}\end{array}\right)|=0$

or, $\left|\begin{array}{ll}{\frac{-a s}{c}-\lambda} & {\frac{-b s}{c}} \\ {\frac{r c-a s}{b}} & {-\lambda}\end{array}\right|=0$

or,$\lambda^{2}+\frac{a s}{c} \lambda+\frac{b s}{c}\left(\frac{r c-a s}{b}\right)=0$

$\therefore \lambda=\frac{-\frac{a s}{c} \pm \sqrt{\left(\frac{a s}{c}\right)^{2}-4 \frac{b s}{c}\left(\frac{r c-a s}{c}\right)}}{2.1}$

$\therefore \lambda_{1}=\frac{-\frac{a s}{c}+\sqrt{\left(\frac{a s}{c}\right)^{2}-4 \frac{b s}{c}\left(\frac{r c-a s}{c}\right)}}{2.1}$

and, $\lambda_{2}=\frac{-\frac{a s}{c}-\sqrt{\frac{a^{2} s^{2}-4 s r c^{2}+4 a c s^{2}}{c^{2}}}}{2}$

Here, the eigenvalues have negative real parts. So, the equilibrium point $\left(\frac{s}{c}, \frac{r c-a s}{b c}\right)$ is asymptotically stable.

Also, we check stability at the equilibrium point $\left(\frac{r}{a}, 0\right)$ , the Jacobian matrix becomes

$J\left(\frac{r}{a}, 0\right)=\left(\begin{array}{cc}{r-2 \frac{r}{a}} & {-\frac{b r}{a}} \\ {0} & {-s+\frac{r c}{a}}\end{array}\right)$

Now the characteristic equation of Jacobian matrix is

$|J-\lambda I|=0$

or, $\left|\left(\begin{array}{cc}{\frac{r a-2 r}{a}} & {-\frac{b r}{a}} \\ {0} & {\frac{c r-s a}{a}}\end{array}\right)-\left(\begin{array}{ll}{\lambda} & {0} \\ {0} & {\lambda}\end{array}\right)\right|=0$

or, $| \left(\begin{array}{cc}{\frac{r a-2 r}{a}} & {-\frac{b r}{a}} \\ {a} & {\frac{c r-s a}{a}}\end{array}\right)-\left(\begin{array}{ll}{\lambda} & {0} \\ {0} & {\lambda}\end{array}\right) |=0$

or,$\left(\frac{r a-2 r}{a}-\lambda\right)\left(\frac{c r-s a}{a}-\lambda\right)=0$

or,$\lambda=\frac{c r-s a}{a}, \frac{r a-2 r}{a}$      (17)

The eigenvalues $\lambda=\frac{c r-s a}{a}$ . $\frac{r a-2 r}{a}$ are surely real numbers.

 Conditions for stability of eigenvalues

 $\lambda=\frac{c r-s a}{a}, \frac{r a-2 r}{a}$

i. If $c r<s a$ and $a<2$ both are negative, then the equilibrium point $\left(\frac{r}{a}, 0\right)$ is asymptotically stable.

ii. If $c r<s a$ and $a<2$ both are positive, then the equilibrium point $\left(\frac{r}{a}, 0\right)$ is unstable.

iii. If $c r<s a$ is negative and $a<2$ is positive, then the equilibrium point $\left(\frac{r}{a}, 0\right)$ is unstable.

iv. If $c r<s a$ is positive and $a<2$ is negative, then the equilibrium point $\left(\frac{r}{a}, 0\right)$ is unstable.

Since, the Jacobian matrix at equilibrium point $\left(x^{*}, y^{*}\right)$ is

$J\left(x^{*}, \mathrm{y}^{*}\right)=\left(\begin{array}{cc}{r-2 a x-b y} & {-b x} \\ {c y} & {-s+c x}\end{array}\right)$

and the characterisic equation of the above Jacobian matrix is $\lambda^{2}+b_{1} \lambda+b_{2}=0$ where $b_{1}=2 a x^{*}+\left(s-c x^{*}\right)+\left(b y^{*}-r\right)$ ,$b_{2}=2 a x^{*}\left(s-c x^{*}\right)+\left(b y^{*}-r\right) s+c r x^{*}$

For $b_{1}$ , $b_{2}>0$ , according to Hurwitz’s theorem, the real parts of all roots of the characteristic equation must be negative. i.e. asymptotically stable.

There are three equilibrium points (0,0) , $\left(\frac{s}{c}, \frac{r c-a s-d}{b c}\right)$ and $\left(\frac{r-d}{a}, 0\right)$ of the following guava model.

$\frac{d x}{d t}=r x-a x^{2}-b x y-d x=f(x, y)$

$\frac{d y}{d t}=-s y+c x y=g(x, y)$

For equilibrium point of the model,

$\frac{d x}{d t}=0$ , $\frac{d y}{d t}=0$

To obtain equilibrium point, we have to solve the algebraic equations.

$r x-a x^{2}-b x y-d x=0$

$-s y+c x y=0$

Let $\left(x^{*}, y^{*}\right)$ be the equilibrium points.

Hence,

$f\left(x^{*}, y^{*}\right)=r x^{*}-a\left(x^{*}\right)^{2}-b x^{*} y^{*}-d x^{*}=0$     (20)    

$g\left(x^{*}, y^{*}\right)=-s y^{*}+c x^{*} y^{*}=0$     (21)   

From (21) we get,

$-s y^{*}+c x^{*} y^{*}=0$

or, $y^{*}\left(-s+c x^{*}\right)=0$

or, $y^{*}=0$ and $-s+c x^{*}=0$

                 $\therefore \mathrm{y}^{*}=0$ and  $x^{*}=\frac{S}{c}$       (22)

Again, from (20) we get,

$r x^{*}-a\left(x^{*}\right)^{2}-b x^{*} y^{*}-d x^{*}=0$

or, $x^{*}\left(r-a x^{*}-b y^{*}-d\right)=0$

               or , $x^{*}=0$ and $r-a x^{*}-b y^{*}-d=0$       (23)  

Putting the value of $x^{*}$ from (22) in (23)

$r-a \frac{S}{c}-b y^{*}-d=0$

           $\therefore y^{*}=\frac{r c-a s-d c}{b c}$    (24)

Putting the value of $y^{*}$ from (24) in (23), we get

$r-a x^{*}-d=0$

 $\therefore x^{*}=\frac{r-d}{a}$

Hence, the equilibrium points $\left(x^{*}, y^{*}\right)$ are (0,0) , $\left(\frac{s}{c}, \frac{r c-a s-d c}{b c}\right)$ and $\left(\frac{r-d}{a}, 0\right)$

In this section, we have numerically solved the model and discussed the stability of this model. We have also discussed about the model for different initial values of guava  borers and natural enemies. Here, we have shown the numerical results of the modified model and also discussed the stability.

Firstly, we have solved the model to calculate the numerical results. We estimate about 120-140 days which are required to be a mature guava fruit from its flower. So we have taken the time interval from 0-120 days and 0-140 days.  For the numerical simulations of interaction between the guava borers and its natural enemies, we use the following values of the model coeffecients: $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $S=0.5$ [12]. We use MATLAB (R2010a) to simulate the numerical results of this model by Runge-kutta method.  

1.png

Figure 1. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=5000$ and $y_{0}=500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 140

2.png

Figure 2. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values  $x_{0}=5000$  and $y_{0}=500$ when the time interval from 0 to 140

In Figure 1, we consider the initial values of guava borers ( $x$ ) is 5000 and natural enemies ( $y$ ) is 500 where, $x$ denotes the number of guava borers and $y$ denotes the number of natural enemies. Here, we consider the time interval as days from 0 to 140. When $t=10$ , the figure shows the populations of borers are 1000 and population of natural enemies are 7000. That means the population of natural enemies increase and the population of guava borers decreases. Again at $t=40$ , the value of $x$ is 4500 and the value of $y$ is 100. That means, $y$ is decreasing as well as $x$ is increasing. It is observed that, at $t=0$ , the guava borers ( $x$ ) are 5000 and at $t=40$ , the guava borers ( $x$ ) are 4500. So, we can say that, the number of borer is gradually decreasing. But at terminal point when $t=140$ , borers population is 1.42×10³ and natural enemy’s population is 1.25×10³ . Finally, it is seen that, the number of borers are greater than the number natural enemies.

Here we see that, Figure 3 is similar to Figure 1 in 120 days. In the above figure (Figure 3) we take the time interval from 0 to 120. When, $t=120$ the borer’s population is $x$ = 3.43×10³ and the population of natural enemies are $y$ = 268.3166 .

3.png

Figure 3. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=5000$ and $y_{0}=500$  when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 120

4.png

Figure 4. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=5000$ and $y_{0}=500$ when the time interval from 0 to 120

If we compare the figures 1 and 3, we see that at 120^th day (Figure 3) the borers population ( $x$ ) is much larger than the natural enemy’s population ( $y$ ). At 140^th day (Figure 1) the value of $x$ and $y$ are closer than the value of $x$ and $y$ in Figure 3. That means it is more beneficial for the farmers to pluck the fruit at 140^th day. Because at 140^th day (Figure 1), the number of guava borers are less than the number of guava borers at 120^th day (Figure 3).              

Figure 2 and 4 is the phase portrait of the model for 140 and 120 days respectively. The behavior of all parameter values is oscillatory. We observe that the solutions are periodic and the periodic solutions are orbitally stable. The periodic time in Figure 2 is less than the periodic time in Figure 4. So the growth rate of guava is better for 0 to 120 days than the interval 0 to140 days. But without natural treatment, the more stable situation for the interval 0 to 140 days. The guava borers gradually decrease and increase with respect to natural enemies. The area in the middle of the figure shows the asymptotically stable region.

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Figure 5. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=4500$ and $y_{0}=500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046 a$ , $c=0.0002$ and $S=0.5$ . Here we have taken the time interval from 0 to 140

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Figure 6. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=4500$ and $y_{0}=500$ when the time interval from 0 to 140

Figure 5 is started with the values $x_{0}=4500$ and $y_{0}=500$ . At 10^th day, we get $x=3000$ and $y=6000$ which shows that natural enemy’s population increases rapidly and guava borer’s population rapidly decreases. But, at 20^th day, the value of $x$ is 1300 and $y$ is 500. This means in lack of guava borers the natural enemies decrease. When the natural enemies decrease the borer’s populations again increase. So the figure shows the competition between pest and natural enemies. At 140^th day, $x$ = 1.71 × 10³ and $y$ = 617.9337 Finally, the populations of guava borers are larger than the populations of natural enemies.     

Figure 7 is similar to Figure 5 for the time interval 120 days. We have taken the time interval 0 to 120 in Figure 7. At $t=120$ , we get $x$ = 3.813 ×10³ and $y$ = 911.4765

7.png

Figure 7. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=4500$ and $y_{0}=500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 120

8.png

Figure 8. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=4500$ and $y_{0}=500$ when the time interval from 0 to 120

Comparing Figures 5 and 7, we get that Figure 7 shows the better positions of $x$ and $y$. At 140^th day, $x$ = 1.71 × 10₃ and $x$ = 3.813 × 10³ at $t=120$ . So we observe that if the farmers collect the fruits at 140^th day, they will be benefitted.

In Figure 9, at $t=40$ , we have got $x=5000$ and  $y=10$  But at $t=50$ , we get $x=3000$ and $y=7000$ . That means when $x$ value increases, natural enemies also increase. At $t=90$ , we get $x=4800$ and $y=5500$ .That means, when natural enemies increase, borers decrease. Finally, at $t=140$ , the total number of guava borers are $x=1.57 \times 10^{3}$ and natural enemies are $y=3.57 \times 10^{3}$ So, the populations of natural enemies are larger than the population of guava borers. Figure 11 is similar to Figure 9 for 120 days. At $t=120$ ,  we get $x=2.73 \times 10^{3}$ and $y=106.8028$ . Comparing Figure 9 and Figure 11, we have got the best result for 140 days (Figure 9). If the farmers pick up the fruits at 140^th day, they can receive the quality food than other days.  

9.png

Figure 9. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=5500$ and $y_{0}=500$ when $r=0.07$ ， $a=0.0000020$ ，$b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 140

10.png

Figure 10. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=5500$ and $y_{0}=500$ when the time interval from 0 to 140

11.png

Figure 11. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=5500$ and $y_{0}=500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 120

12.png

Figure 12. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=5500$ and $y_{0}=500$ when the time interval from 0 to 140

13.png

Figure 13. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 140

14.png

Figure 14. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=500$ when the time interval from 0 to 140

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Figure 15. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 120

16.png

Figure 16. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=500$ when the time interval from 0 to 120

17.png

Figure 17. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=4500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 140

In Figure 13, at 50^th day we get,  $x=11500$ and $y=2000$ . At $t=100$ , we get  $x=10000$ and $y=6000$ . Hence we see the interaction between guava borers and natural enemies. At time $t=120$ , $x$ value is  1.67 × 10³ and $y$ value is 32.5099 .

Figure 13 and 15 are similar to each other for 120 days. At $t=120$ , we get $x$ = 5.2022 × 10³  and $y$ = 587.5246 .In figure 13, at 50^th day we get, $x$ = 11500 and $y$ = 2000 . At $t=100$ , we get $x$ = 10000 and $y$ = 6000 . Hence we see the interaction between guava borers and natural enemies. At time $t=120$ , $x$ value is 1.67× 10³ and $y$ value is 32.5099 .Comparing Figures 13 and 15, we get better result for 140 days (Figure 13).

In Figure 17, we see that initially borers population are increasing and natural enemies are decreasing. At $t=55$ , we get, $x$ = 1000 and $y$ = 11800. That means again $x$ is decreasing and $y$ is increasing. Finally, at $t=140$ , we get $x$ = 3.376 × 10³ and $y$ = 21.965.Figure 19 is similar to Figure 17 for 120 days. At $t=120$ , we get $x$ = 970.3620 and $y$ = 209.8794 Comparing the Figure 17 and 19, we get better result for 120 days (Figure 19).

18.png

Figure 18. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=4500$ when the time interval from 0 to 140

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Figure 19. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=4500$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 120

20.png

Figure 20. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$) for initial values $x_{0}=500$ and $y_{0}=4500$ when the time interval from 0 to 140

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Figure 21. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=5000$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 140

In Figure 21, we see that initially guava borers population are increasing and natural enemy’s population are decreasing. At $t=55$ , we get, $x$ = 4000 and $y$ = 12000. That means, again $x$ is decreasing and $y$ is increasing. Finally, at $t=140$ , we get $x$ = 3.049 × 10³  and $y$ = 15.555.

22.png

Figure 22. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=5000$ when the time interval from 0 to 140

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Figure 23. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=5000$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 120

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Figure 24. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values  $x_{0}=500$ and $y_{0}=5000$ when the time interval from 0 to 140

Figure 23 is similar to Figure 21. At $t=120$, we get $x$ = 879.2025 and $y$ = 325.3513 .

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Figure 25. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=6000$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 140

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Figure 26. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=6000$ when the time interval from 0 to 140

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Figure 27. Population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values $x_{0}=500$ and $y_{0}=6000$ when $r=0.07$ , $a=0.0000020$ , $b=0.000046$ , $c=0.0002$ and $s=0.5$ . Here we have taken the time interval from 0 to 120

.

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Figure 28. Phase portrait of population of Guava borers ( $x$ ) and natural enemies ( $y$ ) for initial values  $x_{0}=500$ and $y_{0}=6000$ when the time interval from 0 to 120

Comparing the figures 21 and 23, we get better result for 120 days (Figure 23). In Figure 25, we see that initially guava borers population are increasing and natural enemy’s population are decreasing. At $t=57$ , we get,  $x$ = 3500 and $y$ = 12500 . That means again $x$ is decreasing and $y$ is increasing. Finally, at $t=140$ , we get $x$ = 2.464 × 10³ and $y$ = 15.555. We show that, Figure 27 is similar to Figure 25 for 120 days. But we have taken the time interval 0 to120 in Figure 27. At $t=120$ we the values $x$ = 745.5455 and $y$ = 860.5480 . Comparing Figures 25 and 27, we can say that, if the farmers pick off the fruits at 120^th day (Figure 27), they will be benefitted.

Figure 28 and 26 is the phase portrait of the model for 120 and 140 days.  The behavior of all parameters values is oscillatory. We observe that the solutions are periodic and the periodic solutions are orbitally stable. The periodic time in Figure 28 is less than the periodic time in Figure 26. So, we get the better result for 120 days.

Natural treatment (spraying of neem leaves) on guava fruit save fruit from borer and human healh from side effect of chemical while minimizing the cost of treatment in the presence of natural enemies (parasitoids). We observe that it is a good harvesting time at 140 days for some initial values (when the number of initial values of guava borers are greater than the number of initial values of natural enemies) and we also get a good harvesting time at 120 days for some another initial values (when the number of initial values of natural enemy are greater than the number of initial values of guava borer) in our actual model. But if we compare between the actual model and the modified model, we see that, our modified model that means natural treatment on guava is more preferable than the actual model. Because at 120-140 days, the modified model shows the better results for any initial values of guava borer and natural enemy than our actual model. This modified model is not harmful for animals or nature. We believe that the results of the modified model can help the farmer to save the guava borer and it is also health benefitted for human.