[Resolvido] erro no string.format

Pessoal to usando um metodo que aprendi aki mas não está dando certo no insert veja

public partial class Form1 : Form
{

 SqlConnection Conn;
 SqlCommand Comm;
 SqlDataAdapter DtAdapter;
 SqlDataReader DtReader;
 public Form1()
 {
 InitializeComponent();
 }

 private void Form1_Load(object sender, EventArgs e)
 {
 string a1 = "bbb";
 string b2 = "ccc";

 Conn = new SqlConnection("Data Source=CPD06;Initial Catalog=Northwind;Integrated Security=True");
 Conn.Open();
 label2.Text = Conn.State.ToString();

 //Comm = new SqlCommand("Select LastName from Employees", Conn);

 string sql = string.format("INSERT INTO Employees (LastName,FirstName) VALUES('{0}','{1}'", a1, b2);
 Comm.ExecuteNonQuery();

 //DtReader = Comm.ExecuteReader();

 /*while(DtReader.Read())
 {
 label3.Text = DtReader[0]+"\n";
 }*/
 //DtReader.Close();
 Conn.Close();
 }
	}

Como posso fazer esse insert?