Собственный поток для каждого Listbox.Items

Imports System.Threading
Public Class Form1
    Public Delegate Sub delegate_fill_list(list_line As String)
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        For i = 1 To 5
            ListBox1.Items.Add(i)
        Next
    End Sub
 
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim threads As New List(Of Thread)()
        For i As Integer = 0 To ListBox1.Items.Count
            threads.Add(New Thread(AddressOf fill_list))
            threads(i).IsBackground = True
            threads(i).Start()
        Next
    End Sub
 
    Public Sub fill_list()
        For i = 0 To ListBox1.Items.Count - 1
            Dim list1_line As String = ListBox1.Items(i).ToString
            add_line(list1_line * 5)
            Thread.Sleep(1000)
        Next
    End Sub
 
    Public Sub add_line(line As String)
        If ListBox2.InvokeRequired Then
            Invoke(New delegate_fill_list(AddressOf add_line), line)
        Else
            ListBox2.Items.Add(line)
        End If
    End Sub
End Class

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim threads As New List(Of Thread)()
    For i As Integer = 0 To ListBox1.Items.Count-1
        threads.Add(New Thread(AddressOf fill_list))
        threads(i).IsBackground = True
        threads(i).Start(ListBox1.Items(i))
    Next
End Sub
 
Public Sub fill_list(item As Object)
    Dim list1_line As String = item.ToString
    add_line(list1_line * 5)
End Sub
 
Public Sub add_line(line As String)
    If ListBox2.InvokeRequired Then
        Invoke(New delegate_fill_list(AddressOf add_line), line)
    Else
        ListBox2.Items.Add(line)
    End If
End Sub

            If threads(i).IsAlive = False Then
                threads(i).Start(ListBox1.Items(i))
            End If

            If threads(i - 1).IsAlive = False Then
                threads(i).Start(ListBox1.Items(i))
            End If

        For i As Integer = 0 To ListBox1.Items.Count - 1
            Dim wrk_thread As New Thread(AddressOf fill_list)
            wrk_thread.IsBackground = True
            If wrk_thread.IsAlive = False Then
                wrk_thread.Start(ListBox1.Items(i))
            End If
        Next

    Public Sub fill_list(item As Object)
        Dim list1_line As String = item.ToString
        Thread.Sleep(1000)
        add_line(list1_line * 5)
        ' Thread.Sleep(1000)
    End Sub