How to Capture click-event of DropDownList box

[AuntieMom]

The situation:

A GridView Object is populating a DetailsView Object and I have a dropdownlist within the DetailsView. When this page loads there will always be only one record in the dropdownlist object, then the user needs to click the down arrow to repopulate the dropdownlist control.

How can I capture the <clicking> of the dropdownlist in order for me to re-populate the dropdownlist with the corresponding values.

I have:
<asp:UpdatePanel ID="upLD2" runat ="server" >

<ContentTemplate >
<aspropDownList ID="LD2" runat="server" AutoPostBack="True"
DataSourceID="odsLByD"
DataTextField="LD" DataValueField="LCode">
</aspropDownList>
<asp:ObjectDataSource ID="odsLByDi" runat="server"
OldValuesParameterFormatString="original_{0}"
SelectMethod="GetLByEmpID"
TypeName="LBLL">
<SelectParameters>
<asp:ControlParameter ControlID="grdEmp" Name="EmpD" PropertyName="SelectedValue"
Type="String" />
</SelectParameters>
</asp:ObjectDataSource>
</ContentTemplate>

<Triggers>
<asp:AsyncPostBackTrigger ControlID ="LD2" EventName ="SelectInitialItem" />
</Triggers>
</asp:UpdatePanel>

</EditItemTemplate>