FERA discovered the outbreak in the Paddock Wood area last month. It is the first time the exotic beetle pest has been found in the UK.
In addition to those trees already confirmed as being infested, FERA said a further five trees were "highly likely to be infested" with the pest.
The beetle is a widespread pest in China, where it has killed millions of poplar trees that were planted to prevent soil erosion.
The Asian longhorn beetle poses a serious threat to a wide range of broadleaved trees and shrubs such as maple (including sycamore), elm, horse chestnut, willow, poplar and birch as well as some fruit trees.
A FERA statement said that all infested trees and main host tree species within the Kent infestation zone were being felled. But it added that survey work was continuing outside the initial infested zone so it was "still possible that we will find more infested trees".
FERA was due to hold a meeting at Hop Farm this week to outline how the public can help. A leaflet has been distributed to residential properties within a 2km radius of the infestation zone.