Spawning Boss Glitch

import greenfoot.*;

/**
 * The final enemy Mario has to overcome to win the game.
 * Is spawned after Mario collects 25 coins.
 * 
 * @author (Cathy Liu) 
 * @version (06/04/2015)
 */
public class Boss extends AnimatedActor
{
    private int speed_;
    private double spawn_;
    private int orbTimer_;
    public static int health_ = 20;
    private int yDir = 1;
   
    public Boss()
    {
        super();
        speed_ = 2;
        health_ = 20;
        spawn_ = 0.009;
        orbTimer_ = 0;
    }
    
    /**
     * Shows the speed in which the boss is moving.
     * @return boss' speed
     */
    public int getSpeed()
    {
        return speed_;
    }
    
    /**
     * Shows how much health the Boss still has.
     * @return Boss' health.
     */
    public int getHealth()
    {
        return health_;
    }
    
    /**
     * Act - do whatever the Boss wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        // Add your action code here.
        super.act();
        boundary();
        spawn();
        animateBossOpen();
    }   
    
    /**
     * The movements and boundaries of the Boss.
     * When reaching X coordinate of 716, the Boss only moves up and down.
     * Code credit to danpost on greenfoot.org
     */
    public void boundary() 
    {
        //this code allows the boss to move up and down on the y-axis within a boundary
        if (getX() > 716)// if X is larger than 716, the Boss can still move
        { 
            move(-speed_);
        }
        else
        { // move vertically
            setLocation(getX(), getY()+yDir*speed_);
            if ((yDir > 0 && getY() >= 465) || (yDir < 0 && getY() <= 237))
            { // reverse directions
                yDir = -yDir;
            }
        }
    }
    
    /**
     * The Boss spawns orbs that attacks Mario.
     */
    public void spawn()
    {
        if(Math.random() < spawn_)
        {   
            animateBossOpen();
            
            GreenfootImage BossImg = new GreenfootImage("boss0.gif");
            World w = getWorld();
            
            Orb orb = new Orb(w.getWidth() + BossImg.getWidth()/2, getY());
            w.addObject(orb, getX() - BossImg.getWidth()/2 - getImage().getWidth()/2, getY());
            
            orbTimer_ = 12;
            Greenfoot.playSound("orbfire.wav");
        }
    }
}