Mathematical Problems in Engineering

Mathematical Problems in Engineering / 2012 / Article

Research Article | Open Access

Volume 2012 |Article ID 726783 | 14 pages | https://doi.org/10.1155/2012/726783

Impulsive Biological Pest Control Strategies of the Sugarcane Borer

Academic Editor: Alexander P. Seyranian
Received07 Apr 2012
Accepted11 Jun 2012
Published19 Jul 2012

Abstract

We propose an impulsive biological pest control of the sugarcane borer (Diatraea saccharalis) by its egg parasitoid Trichogramma galloi based on a mathematical model in which the sugarcane borer is represented by the egg and larval stages, and the parasitoid is considered in terms of the parasitized eggs. By using the Floquet theory and the small amplitude perturbation method, we show that there exists a globally asymptotically stable pest-eradication periodic solution when some conditions hold. The numerical simulations show that the impulsive release of parasitoids provides reliable strategies of the biological pest control of the sugarcane borer.

1. Introduction

One of the challenges for the improvements in the farming and harvesting of cane is the biological pest control. Biological control is defined as the reduction of pest populations by using their natural enemies: predators, parasitoids, and pathogens [1]. Parasitoids are species which develop within or on the host and ultimately kill it. Thus, parasitoids are commonly reared in laboratories and periodically released in high-density populations as biological control agents of crop pests [2].

The sugarcane borer Diatraea saccharalis is reported to be the most important sugarcane pest in the southeast region of Brazil [3]. The sugarcane borer builds internal galleries in the sugarcane plants causing direct damage that results in apical bud death, weight loss, and atrophy. Indirect damage occurs when there is contamination by yeasts that cause red rot in the stalks, either causing contamination or inverting the sugar, increasing yield loss in both sugar and alcohol [4].

There is an important larvae parasitoid of the sugarcane borer, a wasp named Cotesia flavipes which is widely used in biological control in Brazil [3]. In spite of this control being considered successful in Brazil, there are some areas where Cotesia flavipes does not control the sugarcane borer efficiently. The using of the egg parasitoid Trichogramma galloi is considered an interesting option in this case [5].

Mathematical modelling is an important tool used in studying agricultural problems. Thus, a good strategy of biological pest control, based on mathematical modelling, can increase the ethanol production. The applications of host-parasitoid models for biological control were reviewed in [6].

In [7], a mathematical model of interaction between the sugarcane borer (Diatraea saccharalis) and its egg parasitoid Trichogramma galloi was proposed which consists of three differential equations 𝑑π‘₯1ξ‚€π‘₯𝑑𝑑=π‘Ÿ1βˆ’1𝐾π‘₯1βˆ’π‘š1π‘₯1βˆ’π‘›1π‘₯1βˆ’π›½π‘₯1π‘₯2,𝑑π‘₯2𝑑𝑑=𝛽π‘₯1π‘₯2βˆ’π‘š2π‘₯2βˆ’π‘›2π‘₯2,𝑑π‘₯3𝑑𝑑=𝑛1π‘₯1βˆ’π‘š3π‘₯3βˆ’π‘›3π‘₯3,(1.1) where π‘₯1 is the egg density of the sugarcane borer, π‘₯2 is the density of eggs parasitized by Trichogramma galloi, and π‘₯3 is the larvae density of the sugarcane borer; π‘Ÿ is the net reproduction rate; 𝐾 is the carrying capacity of the environment; π‘š1, π‘š2, and π‘š3 are mortality rates of the egg, parasitized egg, and larvae populations; 𝑛1 is the fraction of the eggs from which the larvae emerge at time 𝑑; 𝑛2 is the fraction of the parasitized eggs from which the adult parasitoids emerge at time 𝑑; 𝑛3 is the fraction of the larvae population which moults into pupal stage at time 𝑑; 𝛽 is the rate of parasitism. The dynamics of this model without control was considered in [7].

The proposed model (1.1) is a simplified compartmental one which considers only three easy monitoring stages of the sugarcane borer species: egg stage, parasitized egg stage, and larvae stage. Then, a reducing effect related to the searching efficiency of the adult parasitoid may be ignored by this model. According to [5], the searching time of the parasitoid Trichogramma galloi is 1-2 day, and it can cause some augmentation of the parasitoid egg numbers when biological control measures are implemented.

Meanwhile, many authors have investigated the different population models concerning the impulsive pest control [8–15]. The impulsive pest control strategies based on prey-predator models were presented in [8, 11, 13]. The host-parasitoid model with impulsive control was considered in [10]. Impulsive strategies of a pest management for SI epidemic models were proposed in [9, 12]. Pulse vaccination strategies for SIR epidemic models were considered in [14, 15].

In this paper, we suggest impulsive differential equations [16] to model the process of the biological pest control of the sugarcane borer. So we develop (1.1) introducing a periodic releasing of the parasitoids at fixed times 𝑑π‘₯1ξ‚€π‘₯𝑑𝑑=π‘Ÿ1βˆ’1𝐾π‘₯1βˆ’π‘š1π‘₯1βˆ’π‘›1π‘₯1βˆ’π›½π‘₯1π‘₯2𝑑π‘₯2𝑑𝑑=𝛽π‘₯1π‘₯2βˆ’π‘š2π‘₯2βˆ’π‘›2π‘₯2π‘‘β‰ π‘›πœ,π‘›βˆˆπ‘+,𝑑π‘₯3𝑑𝑑=𝑛1π‘₯1βˆ’π‘š3π‘₯3βˆ’π‘›3π‘₯3Ξ”π‘₯1(𝑑)=0Ξ”π‘₯2(𝑑)=𝑝𝑑=π‘›πœ,π‘›βˆˆπ‘+,Ξ”π‘₯3(𝑑)=0,(1.2) where 𝑝 is the release amount of the parasitized eggs at 𝑑=π‘›πœ,π‘›βˆˆπ‘+,𝑍+={0,1,2,…,},𝜏 is the period of the impulsive effect. Ξ”π‘₯𝑖=π‘₯𝑖(𝑑+)βˆ’π‘₯𝑖(𝑑),π‘₯𝑖(𝑑+)=lim𝑑→𝑑+π‘₯𝑖(𝑑),𝑖=1,2,3. That is, we can use releasing parasitized eggs to eradicate pests or keep the pest population below the economic damage level.

2. Preliminary

In this section, we will give some definitions, notations, and some lemmas which will be useful for our main results.

Let 𝑅+=[0,∞),𝑅3+={π‘₯βˆˆπ‘…3∢π‘₯>0}. Denote 𝑓=(𝑓1,𝑓2,𝑓3)𝑇, the map defined by the right-hand side of the first three equations of the system (1.2). Let 𝑉0={π‘‰βˆΆπ‘…+×𝑅3+↦𝑅+} be continuous on (π‘›πœ,(𝑛+1)𝜏]×𝑅3+, lim(𝑑,𝑦)β†’(π‘›πœ+,π‘₯)𝑉(𝑑,𝑦)=𝑉(π‘›πœ+,π‘₯)  exist and 𝑉 is locally Lipschitzian in π‘₯.

Definition 2.1. π‘‰βˆˆπ‘‰0, then for (𝑑,π‘₯)∈(π‘›πœ,(𝑛+1)𝜏]×𝑅3+, the upper right derivative of 𝑉(𝑑,π‘₯) with respect to the impulsive differential system (1.2) is defined as 𝐷+𝑉(𝑑,π‘₯)=limβ„Žβ†’01supβ„Ž[].𝑉(𝑑+β„Ž,π‘₯+β„Žπ‘“(𝑑,π‘₯))βˆ’π‘‰(𝑑,π‘₯)(2.1)

The solution of system (1.2), denoted by π‘₯(𝑑)βˆΆπ‘…+↦𝑅3+, is continuously differentiable on (π‘›πœ,(𝑛+1)𝜏]×𝑅3+. Obviously, the global existence and uniqueness of solution of system (1.2) is guaranteed by the smoothness properties of 𝑓, for details see [16].

We will use a basic comparison result from impulsive differential equations.

Lemma 2.2 (see [16]). Let π‘‰βˆˆπ‘‰0, assume that 𝐷+𝑉𝑑𝑉(𝑑,π‘₯)≀𝑔(𝑑,𝑉(𝑑,π‘₯)),π‘‘β‰ π‘›πœ,𝑑,π‘₯+ξ€Έξ€Έβ‰€πœ“π‘›(𝑉(𝑑,π‘₯(𝑑))),𝑑=π‘›πœ,(2.2) where π‘”βˆΆπ‘…+×𝑅3+↦𝑅+ is continuous on (π‘›πœ,(𝑛+1)𝜏]×𝑅+ and πœ“π‘›βˆΆπ‘…+↦𝑅+ is nondecreasing. Let 𝑅(𝑑) be the maximal solution of the scalar impulsive differential equation 𝑒𝑑̇𝑒(𝑑,π‘₯)=𝑔(𝑑,𝑒(𝑑)),π‘‘β‰ π‘›πœ,+ξ€Έ=πœ“π‘›π‘’ξ€·0(𝑒(𝑑)),𝑑=π‘›πœ,+ξ€Έ=𝑒0(2.3) existing on [0,∞). Then 𝑉(0+,π‘₯0)≀𝑒0 implies that 𝑉(𝑑,π‘₯(𝑑))≀𝑅(𝑑),𝑑β‰₯0, where x(t) is any solution of (1.2), similar results can be obtained when all the directions of the inequalities in the lemma are reversed and πœ“π‘› is nonincreasing. Note that if one has some smoothness conditions of 𝑔 to guarantee the existence and uniqueness of solutions for (2.3), then R(t) is exactly the unique solution of (2.3).

Next, we consider the following system: 𝑑𝑒2𝑑𝑑=π‘Žβˆ’π‘š2𝑒2βˆ’π‘›2𝑒2,π‘‘β‰ π‘›πœ,Δ𝑒2𝑒(𝑑)=𝑏,𝑑=π‘›πœ2ξ€·0+ξ€Έ=𝑒20β‰₯0.(2.4)

Lemma 2.3. System (2.4) has a unique positive periodic solution ̃𝑒2(𝑑) with period 𝜏 and for every solution 𝑒2(𝑑) of (2.4) |𝑒2(𝑑)βˆ’Μƒπ‘’2(𝑑)(𝑑)|β†’0 as π‘‘β†’βˆž, where ̃𝑒2π‘Ž(𝑑)=π‘š2+𝑛2+π‘π‘’βˆ’(π‘š2+𝑛2)(π‘‘βˆ’π‘›πœ)1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏],π‘‘βˆˆ(π‘›πœ,(𝑛+1)𝜏,π‘›βˆˆπ‘+,̃𝑒2ξ€·0+ξ€Έ=π‘Žπ‘š2+𝑛2+𝑝1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏.(2.5)

Proof. Integrating and solving the first equation of (2.4) between pulses, we get 𝑒2π‘Ž(𝑑)=π‘š2+𝑛2+𝑒2ξ€·π‘›πœ+ξ€Έπ‘’βˆ’(π‘š2+𝑛2)(π‘‘βˆ’π‘›πœ)].,π‘‘βˆˆ(π‘›πœ,(𝑛+1)𝜏(2.6)
After each successive pulse, we can deduce the following map of system (2.6): 𝑒2ξ€·(𝑛+1)𝜏+ξ€Έ=π‘Žπ‘š2+𝑛2+𝑒2ξ€·π‘›πœ+ξ€Έβˆ’π‘Žπ‘š2+𝑛2ξ‚Ήπ‘’βˆ’(π‘š2+𝑛2)𝜏]+𝑝,π‘‘βˆˆ(π‘›πœ,(𝑛+1)𝜏.(2.7) Equation (2.7) has a unique fixed point π‘’βˆ—2=π‘Ž/(π‘š2+𝑛2)+𝑝/(1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏), it corresponds to the unique positive periodic solution ̃𝑒2(𝑑) of system (2.4) with the initial value ̃𝑒2(0+)=π‘Ž/(π‘š2+𝑛2) + 𝑝/(1βˆ’π‘’βˆ’(π‘š_2+𝑛_2)𝜏). The fixed point π‘’βˆ—2 of map (2.7) implies that there is a corresponding cycle of period 𝜏 in 𝑒2(𝑑), that is, ̃𝑒2(𝑑)=(π‘Ž/π‘š2+𝑛2)+(π‘π‘’βˆ’(π‘š2+𝑛2)(π‘‘βˆ’π‘›πœ)/(1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏)),π‘‘βˆˆ(π‘›πœ,(𝑛+1)𝜏],π‘›βˆˆπ‘+. From (2.7) we obtain 𝑒2ξ€·π‘›πœ+ξ€Έ=𝑒2ξ€·0+ξ€Έπ‘’βˆ’π‘›(π‘š2+𝑛2)𝜏+ξ‚΅π‘Žπ‘+π‘š2+𝑛2ξ€·1βˆ’π‘’βˆ’(π‘š2+𝑛2)πœξ€Έξ‚Άξ€·1βˆ’π‘’βˆ’π‘›(π‘š2+𝑛2)πœξ€Έ1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏,(2.8) thus, 𝑒2(π‘›πœ+)β†’π‘’βˆ—2 as π‘‘β†’βˆž, so ̃𝑒2(𝑑) is globally asymptotically stable. Thus, we have 𝑒2(𝑑) = (𝑒2(0+)βˆ’Μƒπ‘’2(0+))π‘’βˆ’(π‘š2+𝑛2)𝑑+̃𝑒2(𝑑).

Consequently, 𝑒2(𝑑)→̃𝑒2(𝑑) as π‘‘β†’βˆž, that is, |𝑒2(𝑑)βˆ’Μƒπ‘’2(𝑑)|β†’0 as π‘‘β†’βˆž.

If π‘Ž=0, the system (2.4) has a unique positive periodic solution ̃𝑒2(𝑑)=(π‘π‘’βˆ’(π‘š2+𝑛2)(π‘‘βˆ’π‘›πœ)/(1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏) with initial condition ̃𝑒2(0+)=𝑝/(1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏) and ̃𝑒2(𝑑) is globally asymptotically stable. This completes the proof.

Therefore, system (1.2) has a pest-eradication periodic solution (0,Μƒπ‘₯2(𝑑),0), where Μƒπ‘₯2(𝑑)=𝑝𝑒(βˆ’(π‘š2+𝑛2)(π‘‘βˆ’π‘›πœ))/(1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏).

To study the stability of the pest-eradication periodic solution of (1.2), we present the Floquet theory for a linear 𝜏 periodic impulsive equation 𝑑π‘₯𝑑𝑑=𝐴(𝑑)π‘₯,π‘‘β‰ πœπ‘˜π‘₯𝑑,π‘‘βˆˆπ‘…,+ξ€Έ=π‘₯(𝑑)+π΅π‘˜π‘₯(𝑑),𝑑=πœπ‘˜,π‘˜βˆˆπ‘+.(2.9) Then, we introduce the following conditions:(𝐻1)𝐴(β‹…)βˆˆπ‘ƒπΆ(𝑅,𝐢𝑛×𝑛) and 𝐴(𝑑+𝜏)=𝐴(𝑑)(π‘‘βˆˆπ‘…), where 𝑃𝐢(𝑅,𝐢𝑛×𝑛) is the set of all piecewise continuous matrix functions which is left continuous at 𝑑=πœπ‘˜, and 𝐢𝑛×𝑛 is the set of all 𝑛×𝑛 matrices.(𝐻2)π΅π‘˜βˆˆπΆπ‘›Γ—π‘›,det(𝐸+π΅π‘˜)β‰ 0,πœπ‘˜<πœπ‘˜+1(π‘˜βˆˆπ‘+).(𝐻3) There exist a β„Žβˆˆπ‘+, such that π΅π‘˜+β„Ž=π΅π‘˜, πœπ‘˜+β„Ž=πœπ‘˜+𝜏(π‘˜βˆˆπ‘+).

Let Ξ¦(𝑑) be the fundamental matrix of (2.9), then there exists a unique nonsingular matrix π‘€βˆˆπΆπ‘›Γ—π‘› such that Ξ¦(𝑑+𝜏)=Ξ¦(𝑑)𝑀.(2.10)

By equality (2.10) there correspondents to the fundamental matrix Ξ¦(𝑑) the constant matrix 𝑀 which is called monodromy matrix of (2.9). All monodromy matrices of (2.9) are similar and have the same eigenvalues. The eigenvalues πœ†1,πœ†2,…,πœ†π‘› of the monodromy matrices are called the Floquet multipliers of (2.9).

Lemma 2.4 (Floquet theory [16]). Let conditions (𝐻1-𝐻3) hold. Then the linear 𝜏 periodic impulsive system (2.9) is as follows:(a)stable if and only if all multipliers πœ†π‘– (𝑖=1,2,…,𝑛) of equation (2.9) satisfy the inequality |πœ†π‘–|≀1,(b)asymptotically stable if and only if all multipliers πœ†π‘– (𝑖=1,2,…,𝑛) of equation (2.9) satisfy the inequality |πœ†π‘–|<1,(c)unstable if |πœ†π‘–|>1 for some 𝑖=1,2,…,𝑛.

3. Stability of the Pest-Eradication Periodic Solution

In this section, we study the stability of the pest-eradication periodic solution (0,Μƒπ‘₯2(𝑑),0) of the system (1.2). Next, we present an important result, concerning a condition that guarantees the global stability of this solution.

Theorem 3.1. The pest-eradication periodic solution (0,Μƒπ‘₯2(𝑑),0) of the system (1.2) is globally asymptotically stable provided that inequality 𝑝>π‘Ÿβˆ’π‘š1βˆ’π‘›1π‘šξ€Έξ€·2+𝑛2ξ€Έπœπ›½(3.1) holds.

Proof. The local stability of a periodic solution (0,Μƒπ‘₯2(𝑑),0) of system (1.2) may be determined by considering the behavior of small-amplitude perturbations (𝑦1(𝑑),𝑦2(𝑑),𝑦3(𝑑)) of the solution.
Define π‘₯1(𝑑)=𝑦1(𝑑),π‘₯2(𝑑)=Μƒπ‘₯2(𝑑)+𝑦2(𝑑),π‘₯3(𝑑)=𝑦3(𝑑),(3.2) where 𝑦1(𝑑),𝑦2(𝑑),𝑦3(𝑑) are small perturbations.
Linearizing the system (1.2), we have the following linear 𝜏 periodic impulsive system: 𝑑𝑦1𝑑𝑑=π‘Ÿπ‘¦1βˆ’π‘š1𝑦1βˆ’π‘›1𝑦1βˆ’π›½Μƒπ‘₯2𝑦1𝑑𝑦2𝑑𝑑=𝛽̃π‘₯2𝑦1βˆ’π‘š2𝑦2βˆ’π‘›2𝑦2π‘‘β‰ π‘›πœ,π‘›βˆˆπ‘+,𝑑𝑦3𝑑𝑑=𝑛1𝑦1βˆ’π‘š3𝑦3βˆ’π‘›3𝑦3𝑦1𝑑+ξ€Έ=𝑦1𝑦(𝑑)2𝑑+ξ€Έ=𝑦2(𝑑)𝑑=π‘›πœ,π‘›βˆˆπ‘+,𝑦3𝑑+ξ€Έ=𝑦3(𝑑).(3.3)
Let Ξ¦(𝑑) be the fundamental matrix of (3.3). Then we have βŽ‘βŽ’βŽ’βŽ’βŽ’βŽ£π‘¦1𝑦(𝑑)2(𝑦𝑑)3⎀βŽ₯βŽ₯βŽ₯βŽ₯βŽ¦βŽ‘βŽ’βŽ’βŽ’βŽ’βŽ£π‘¦(𝑑)=Ξ¦(𝑑)1𝑦(0)2(𝑦0)3⎀βŽ₯βŽ₯βŽ₯βŽ₯⎦(0),(3.4) where Ξ¦(𝑑) must satisfy the following equation: 𝑑Φ(𝑑)=βŽ‘βŽ’βŽ’βŽ’βŽ’βŽ£π‘‘π‘‘π‘Ÿβˆ’π‘š1βˆ’π‘›1βˆ’π›½Μƒπ‘₯200𝛽̃π‘₯2βˆ’π‘š2βˆ’π‘›20𝑛10βˆ’π‘š3βˆ’π‘›3⎀βŽ₯βŽ₯βŽ₯βŽ₯⎦Φ(𝑑),(3.5) and initial condition Ξ¦(𝑑)=𝐼,(3.6) where 𝐼 is the identity matrix.
The solution of (3.5) is βŽ‘βŽ’βŽ’βŽ’βŽ’βŽ£ξ‚€βˆ«Ξ¦(𝑑)=exp𝑑0ξ€·π‘Ÿβˆ’π‘š1βˆ’π‘›1βˆ’π›½Μƒπ‘₯2ξ€Έξ‚ξ€·βˆ’ξ€·π‘š(𝑠)𝑑𝑠00βˆ—exp2+𝑛2ξ€Έπœξ€Έ0ξ€·βˆ’ξ€·π‘šβˆ—βˆ—exp3+𝑛3ξ€Έπœξ€ΈβŽ€βŽ₯βŽ₯βŽ₯βŽ₯⎦.(3.7)
There is no need to calculate the exact form of (βˆ—) as it is not required in the analysis that follows. The resetting impulsive condition of (3.3) becomes βŽ‘βŽ’βŽ’βŽ’βŽ’βŽ£π‘¦1ξ€·π‘›πœ+𝑦2ξ€·π‘›πœ+𝑦3ξ€·π‘›πœ+ξ€ΈβŽ€βŽ₯βŽ₯βŽ₯βŽ₯⎦=⎑⎒⎒⎒⎒⎣⎀βŽ₯βŽ₯βŽ₯βŽ₯βŽ¦βŽ‘βŽ’βŽ’βŽ’βŽ’βŽ£π‘¦1000100011𝑦(π‘›πœ)2(π‘¦π‘›πœ)3⎀βŽ₯βŽ₯βŽ₯βŽ₯⎦(π‘›πœ).(3.8)
Hence, if absolute values of all eigenvalues of ⎑⎒⎒⎒⎒⎣⎀βŽ₯βŽ₯βŽ₯βŽ₯βŽ¦π‘€=100010001Ξ¦(𝜏)=Ξ¦(𝜏)(3.9) are less than one, the 𝜏 periodic solution is locally stable. Then the eigenvalues of 𝑀 are the following: πœ†1ξ‚΅ξ€œ=exp𝑑0ξ€·π‘Ÿβˆ’π‘š1βˆ’π‘›1βˆ’π›½Μƒπ‘₯2(ξ€Έξ‚Άπœ†π‘ )𝑑𝑠2ξ€·βˆ’ξ€·π‘š=exp2+𝑛2ξ€Έπœξ€Έπœ†<13ξ€·βˆ’ξ€·π‘š=exp3+𝑛3ξ€Έπœξ€Έ<1.(3.10) From (3.10), one can see that |πœ†1|<1 if and only if condition (3.1) holds true. According to Lemma 2.4, the pest-eradication periodic solution (0,Μƒπ‘₯2(𝑑),0) is locally asymptotically stable.
In the following, we prove the global attractivity. Choose sufficiently small πœ€>0 such that ξ‚΅ξ€œπ›Ώ=exp𝜏0ξ€·π‘Ÿβˆ’π‘š1βˆ’π‘›1ξ€·βˆ’π›½Μƒπ‘₯2ξ‚Ά(𝑑)βˆ’πœ€ξ€Έξ€Έπ‘‘π‘‘<1.(3.11) From the second equation of system (1.2), noting that ((𝑑π‘₯2)/𝑑𝑑)β‰₯βˆ’(π‘š2+𝑛2)π‘₯2, we consider the following impulsive differential equation 𝑑𝑒2ξ€·π‘šπ‘‘π‘‘=βˆ’2+𝑛2𝑒2,π‘‘β‰ π‘›πœΞ”π‘’2𝑒(𝑑)=𝑝,𝑑=π‘›πœ,2ξ€·0+ξ€Έ=π‘₯2ξ€·0+ξ€Έ.(3.12) From Lemma 2.3, system (3.12) has a globally asymptotically stable positive periodic solution ̃𝑒2(𝑑)=π‘π‘’βˆ’(π‘š2+𝑛2)(π‘‘βˆ’π‘›πœ)1βˆ’π‘’βˆ’(π‘š2+𝑛2)𝜏=Μƒπ‘₯2],π‘‘βˆˆ(π‘›πœ,(𝑛+1)𝜏,π‘›βˆˆπ‘+.(3.13) So by Lemma 2.2, we get π‘₯2(𝑑)β‰₯𝑒2(𝑑)β‰₯Μƒπ‘₯2(𝑑)βˆ’πœ€,(3.14) for all 𝑑 large enough.
From system, (1.2) and (3.14), we obtain that 𝑑π‘₯1ξ‚€π‘₯π‘‘π‘‘β‰€π‘Ÿ1βˆ’1𝐾π‘₯1βˆ’π‘š1π‘₯1βˆ’π‘›1π‘₯1βˆ’π›½π‘₯1ξ€·Μƒπ‘₯2ξ€Έβˆ’πœ€,π‘‘β‰ π‘›πœ,Ξ”π‘₯1(𝑑)=0,𝑑=π‘›πœ.(3.15) Integrating (3.15) on (π‘›πœ,(𝑛+1)𝜏], we get π‘₯1ξ€·(𝑛+1)𝜏+≀π‘₯1ξ‚΅ξ€œ(π‘›πœ)exp𝜏0ξ€·π‘Ÿβˆ’π‘š1βˆ’π‘›1ξ€·βˆ’π›½Μƒπ‘₯2ξ‚Ά(𝑑)βˆ’πœ€ξ€Έξ€Έπ‘‘π‘‘=π‘₯1(π‘›πœ)𝛿.(3.16) Thus, π‘₯1(π‘›πœ)≀π‘₯1(0+)𝛿𝑛 and π‘₯1(π‘›πœ)β†’0 as π‘›β†’βˆž. Therefore, π‘₯1(𝑑)β†’0 as π‘›β†’βˆž, since 0<π‘₯1(𝑑)≀π‘₯1(π‘›πœ) for π‘‘βˆˆ(π‘›πœ,(𝑛+1)𝜏],π‘›βˆˆπ‘+.
Next, we prove that π‘₯2(𝑑)β†’Μƒπ‘₯2(𝑑) as π‘‘β†’βˆž. For 0<πœ€1β‰€π‘š2+𝑛2, there must exist a 𝑑0>0 such that 0<π‘₯1(𝑑)β‰€πœ€1 for all 𝑑β‰₯𝑑0. Without loss of generality, we may assume that 0<π‘₯1(𝑑)β‰€πœ€1 for all 𝑑β‰₯0, from system (1.2) we have 𝑑π‘₯2β‰€ξ€Ίπœ€π‘‘π‘‘1βˆ’ξ€·π‘š2+𝑛2π‘₯ξ€Έξ€»2.(3.17)
Then, we have π‘₯2(𝑑)≀𝑣2(𝑑), while 𝑣2(𝑑)is the solution of 𝑑𝑣2=ξ€Ίπœ€π‘‘π‘‘1βˆ’ξ€·π‘š2+𝑛2𝑣2,π‘‘β‰ π‘›πœ,Δ𝑣2𝑣(𝑑)=𝑝,𝑑=π‘›πœ,2ξ€·0+ξ€Έ=π‘₯2ξ€·0+ξ€Έ.(3.18) By Lemma 2.3, system (3.18) has a positive periodic solution ̃𝑣2(𝑑)=𝑝𝑒[πœ€1βˆ’(π‘š2+𝑛2)](π‘‘βˆ’π‘›πœ)1βˆ’π‘’[πœ€1βˆ’(π‘š2+𝑛2)]𝜏],π‘‘βˆˆ(π‘›πœ,(𝑛+1)𝜏,π‘›βˆˆπ‘+.(3.19) Therefore, for any πœ€2>0, there exists a 𝑑1,𝑑>𝑑1 such that π‘₯2(𝑑)≀𝑣2̃𝑣(𝑑)<2(𝑑)+πœ€2.(3.20) Combining (3.14) and (3.20), we obtain Μƒπ‘₯2(𝑑)βˆ’πœ€β‰€π‘₯2̃𝑣(𝑑)<2(𝑑)+πœ€2,(3.21) for 𝑑 large enough. Let πœ€1,πœ€2β†’0, we get ̃𝑣2(𝑑)β†’Μƒπ‘₯2(𝑑), then π‘₯2(𝑑)β†’Μƒπ‘₯2(𝑑) as π‘‘β†’βˆž.
Assuming that 0<π‘₯1(𝑑)β‰€πœ€1 for all 𝑑β‰₯0, from system (1.2) we have 𝑑π‘₯3𝑑𝑑≀𝑛1πœ€1βˆ’ξ€·π‘š3+𝑛3ξ€Έπ‘₯3.(3.22) Then, we have π‘₯3(𝑑)≀𝑣3(𝑑), while 𝑣3(𝑑) is the solution of the following system: 𝑑𝑣3𝑑𝑑=𝑛1πœ€1βˆ’π‘š3𝑣3βˆ’π‘›3𝑣3,π‘‘β‰ π‘›πœ,Δ𝑣3𝑣(𝑑)=0,𝑑=π‘›πœ,3ξ€·0+ξ€Έ=𝑣30β‰₯0.(3.23) By Lemma 2.3, system (3.23) has a positive solution ̃𝑣3=𝑛1πœ€1π‘š3+𝑛3.(3.24) Thus, for any πœ€3>0, there exists a 𝑑1,𝑑>𝑑1 such that π‘₯3(𝑑)≀𝑣3̃𝑣(𝑑)<3(𝑑)+πœ€3.(3.25) Let πœ€1,πœ€3β†’0, we get ̃𝑣3(𝑑)β†’0, then π‘₯3(𝑑)β†’0 as π‘‘β†’βˆž. This completes the proof.

4. Numerical Simulations of the Impulsive Biological Control

For numerical simulations of interactions between the sugarcane borer and its parasitoid the following values of model coefficients were used: 𝑛1=0.1, 𝑛2=0.1, 𝑛3=0.02439, π‘š1=0.03566, π‘š2=0.03566, π‘š3=0.00256, 𝐾=25000. These values were obtained based on data published about the use of the egg parasitoid Trichogramma galloi against the sugarcane borer Diatraea saccharalis [3, 5, 7]. Figure 1 shows the population oscillations for π‘Ÿ=0.1908 and 𝛽=0.0001723 without control.

According to [10], economic injury level (EIL) cause economic damage. Economic threshold (ET) is population density at which control measures should be determined to prevent an increasing pest population from reaching the economic injury level. One can see from Figure 1 that the sugarcane borer larvae density π‘₯3 takes on values more than the EIL for this pest π‘₯EIL=2500 numbers/ha [3]. In this case, it is necessary to apply the biological control.

From Theorem 3.1, we have shown that the pest-eradication periodic solution (0,Μƒπ‘₯2(𝑑),0) of the system (1.2) is globally asymptotically stable if the condition (3.1) holds 𝑝>𝑝min=ξ€·π‘Ÿβˆ’π‘š1βˆ’π‘›1π‘šξ€Έξ€·2+𝑛2ξ€Έπœπ›½.(4.1) Choosing 𝜏=70 days from (4.1), we derive that when 𝑝>𝑝min=3027 parasitoids/ha, the pest-eradication periodic solution of the host-parasitoid system is asymptotically stable. Dynamical behavior of the system with impulsive control 𝑝=3500 parasitoids/ha and with economic threshold π‘₯ET=2000 is shown in Figure 2. We can conclude that this control strategy seems successful because the larvae population of the sugarcane borer goes to extinction. But the aim of the biological control is not to eliminate all larvae population. The aim of the biological control of the sugarcane borer is to keep the larvae population at an acceptable low level (below the EIL) that indicates the pest densities at which applied biological control is economically justified.

Choosing the release amount 𝑝=1500 parasitoids/ha, we can control the larvae population and keep it below the EIL (see Figure 3). It is obvious that the cost of the control strategy 𝑝=1500 is less than the cost of 𝑝=3500.

Applying the control strategy 𝑝=1000, we can see that the number of larvae individuals exceed π‘₯EIL at some time (see Figure 4).

5. Discussion and Conclusion

In this paper, we suggest a system of impulsive differential equations to model the process of the biological control of the sugarcane borer by periodically releasing its parasitoids. By using the Floquet theory and small amplitude perturbation method, we have proved that for any fixed period 𝜏 there exists a globally asymptotically stable pest-eradication periodic solution (0,Μƒπ‘₯2(𝑑),0) of the system (1.2) if the number of the parasitoids in periodic releases is greater than some critical value 𝑝min.

When the stability of the pest-eradication periodic solution is lost, the numerical results show that the system (1.2) has rich dynamics.

If we choose the biological control strategy by periodical releases of the constant amount of parasitoids, the results of Theorem 3.1 can help in designing the control strategy by informing decisions on the timing of parasitoid releases. In this case, from (3.1) we have 𝜏<𝜏max=π›½π‘ξ€·π‘Ÿβˆ’π‘š1βˆ’π‘›1π‘šξ€Έξ€·2+𝑛2ξ€Έ.(5.1) From (5.1) we can conclude that there exists a globally asymptotically stable pest-eradication periodic solution (0,Μƒπ‘₯2(𝑑),0) of the system (1.2) if the impulsive period is less than some critical value 𝜏max.

It is interesting to discuss the result of Theorem 3.1, comparing the condition of the pest-free globally stable solution (3.1) with the similar result presented in [10]. In their paper, the authors considered integrated pest management strategies based on discrete-time host-parasitoid models. Integrated pest management (IPM) is a long-term control strategy that combines biological, cultural, and chemical tactics to reduce pest populations to tolerable levels when the pests reach the ET [17]. IPM control strategy is not used in sugarcane crops because it is impossible to kill the sugarcane borer larvae by insecticides when it builds internal galleries in the sugarcane plants. The biological control is unique strategy of the pest control in this case.

The resetting impulsive condition (3.1) which guarantees the global stability of the host-eradication periodic solution becomes π‘Ÿβˆ’π‘š1βˆ’π‘›1<π›½π‘πœξ€·π‘š2+𝑛2ξ€Έ,(5.2) which means that if the intrinsic growth rate is less than the mean parasitization rate over period 𝜏, then the host population will become extinct eventually. This is the same conclusion which Tang et al. presented in [10] based on the inequality (3.9) from [10]. Then, the inequality (5.2) for the continuous-time model (1.2) and the inequality (3.9) for the discrete-time host-parasitoid model from [10] lead to similar results.

Thus, the results of the present study show that the impulsive release of the parasitoids provides reliable strategies of the biological pest control of the sugarcane borer.

Acknowledgments

The authors would like to thank the referees for their careful reading of the original paper and their valuable comments and suggestions that improved the presentation of this paper. The first author thanks Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Pesquisas (CNPq) for the financial supports on this research.

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Copyright © 2012 Marat Rafikov et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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