Duckweed can be brought to your pond by bird migration.  Seeds can be transplanted by the birds feet.  Duckweed can be very detremental to the health of your pond or lake simply by sheilding the much needed sun rays.  Sun helps produce oxygen for our pond through photosynthesis. Duckweed can be treated. Some of the more effective treatments can be expensive.  The most effectiive treatment is using Fluridone in a liquid form for the treatment of Duckweed. Fluridone need to get into the water column.  It is not designed to spray on the duckweed.  The sun breaks down the chemical too quickly and will not be effective.