How can I get an UpdatePanel to intercept a CompositeControl's DropDownList

<asp:UpdatePanel ID="UpdatePanel" runat="server">
    <ContentTemplate>
        <MyControl:Control ID="CustomControl" runat="server" />
    </ContentTemplate>
</asp:UpdatePanel>

country.SelectedIndexChanged += new EventHandler(country_SelectedIndexChanged);

protected void country_SelectedIndexChanged(Object sender, EventArgs e)
{
    states.DataSource = XXX;
    states.DataBind();
}

public partial class CatchMyEvent : System.Web.UI.UserControl
{
    public delegate void ChangedIndex(object sender, EventArgs e);
    public event ChangedIndex SelectedIndexChanged;

    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
        dropDownListThrow.SelectedIndexChanged += new EventHandler(dropDownListThrow_SelectedIndexChanged);
        labelOutput.Text = "no";
    }

    public void dropDownListThrow_SelectedIndexChanged(object sender, EventArgs e)
    {
        labelOutput.Text = ((DropDownList)sender).SelectedItem.Text;
        if(SelectedIndexChanged != null)
        {
            SelectedIndexChanged(sender, e);
        }
    }
}

<asp:AsyncPostBackTrigger ControlID="catchMyEventMain" EventName="SelectedIndexChanged" />

protected override void OnInit(EventArgs e)
{
    base.OnInit(e);
    catchMyEventMain.SelectedIndexChanged += dropDownListThrow_SelectedIndexChanged;
}

public void dropDownListThrow_SelectedIndexChanged(object sender, EventArgs e)
{
    labelSelectedValue.Text = ((DropDownList)sender).SelectedItem.Text;
}

...
        if (DesignMode || Page == null) return;

        var sm = ScriptManager.GetCurrent(Page);
        if (sm == null)
        {
            throw new MissingFieldException("The ScriptManager is needed on the page!");
        }
        sm.RegisterAsyncPostBackControl(<control which initiates async postback>);