Call web services on some condition

Dear all expert,

I have a DropDownList and Textbox. 

<asp:TextBox ID="txtEditSearch" runat="server" Width="280%" Height="30px" OnTextChanged="txtEditSearch_TextChanged"
                                                AutoPostBack="True"></asp:TextBox><cc1:AutoCompleteExtender ID="AutoEdit" runat="server" CompletionInterval="100" CompletionSetCount="12"
                                                DelimiterCharacters="" Enabled="True" MinimumPrefixLength="1" ServiceMethod="DairyDate"
                                                ServicePath="~/WebService.asmx" TargetControlID="txtEditSearch"></cc1:AutoCompleteExtender>

I am using Web services method on this textbox.now problem is that if DDl is a --Select-- position still my webservices call, now i want.if DDL position value is 0(--Select--) then webservices method not call.