How to disable auto open sub-menu in Accordion Pane?

Here is my aspx code:

<asp:Accordion ID="acdBackIssue" runat="server" Height="90px" Width="223px" HeaderCssClass="accordionHeader"
                            HeaderSelectedCssClass="accordionHeaderSelected" SuppressHeaderPostbacks="True"
                            RequireOpenedPane="false"><Panes><asp:AccordionPane ID="AccordionPane1" runat="server" ContentCssClass="backIssueBG"
                                    HeaderCssClass="BI2010"><Header>
                                        2010s Back Issues</Header><Content><ul><asp:Literal ID="ltlBackIssue2K10" runat="server"></asp:Literal></ul></Content></asp:AccordionPane><asp:AccordionPane ID="AccordionPane2" runat="server" ContentCssClass="backIssueBG"
                                    HeaderCssClass="BI2000"><Header>
                                        2000s Back Issues</Header><Content><ul><asp:Literal ID="ltlBackIssue2K" runat="server"></asp:Literal><li><a href="backIssues/Winter2006_v3/">Winter 2006 v3</a></li><li><a href="backIssues/Winter2006_v3/summer2006.aspx">Summer 2006</a></li><li><a href="backIssues/Winter2006_v3/winter2006.aspx">Winter 2006</a></li><li><a href="backIssues/spring04/">Spring 2004</a></li><li><a href="backIssues/Winter2006_v3/winter2004.aspx">Winter 2004</a></li><li><a href="backIssues/spring02/">Spring 2002</a></li><li><a href="backIssues/fall01/">Fall 2001</a></li><li><a href="backIssues/spring01/">Spring 2001</a></li><li><a href="backIssues/fall00/">Fall 2000</a></li><li><a href="backIssues/spring00/">Spring 2000</a></li></ul></Content></asp:AccordionPane><asp:AccordionPane ID="AccordionPane3" runat="server" ContentCssClass="backIssueBG" HeaderCssClass="BI1990"><Header>
                                        1990s Back Issues</Header><Content><ul><li><a href="backIssues/fall99/">Fall 1999</a></li><li><a href="backIssues/spring99/">Spring 1999</a></li><li><a href="backIssues/spring98/">Spring 1998</a></li><li><a href="backIssues/winter98/">Winter 1998</a></li><li><a href="backIssues/fall97/">Fall 1997</a></li><li><a href="backIssues/spring97/">Spring 1997</a></li><li><a href="backIssues/winter97/">Winter 1997</a></li><li><a href="backIssues/fall96/">Fall 1996</a></li><li><a href="backIssues/spring96/">Spring 1996</a></li><li><a href="backIssues/winter96/">Winter 1996</a></li><li><a href="backIssues/fall95/">Fall 1995</a></li></ul></Content></asp:AccordionPane></Panes></asp:Accordion>

And here's a part of the menu that I like to change.

The 2010s Back Issues menu or pane automatically showed its submenus on page load. How do I disable this? Is it in the CSS or the Accordion control itself? I only want to show the sub-menu if when the user clicks on the menu (2010s Back Issues).