In weed management, biocontrol rarely kills a weed population outright, but instead reduces weed vigour and seed output. Hence it is great for:
Big patches that you can’t possibly get rid of ever
Hard to access patches that you can’t reach to treat before seeds get produced each year.
Situations where chemical treatment is unaffordable
Places where it is hard to treat with chemical without getting lots of off target damage
Note that in weed management, biocontrol relies on specificity between weed and agent. The agent must be specific to the weed and not able to survive by eating other plants. In Australia agents undergo very strict specificity testing before being introduced to control weeds, to ensure no off-target damage occurs. No one wants a repeat of the cane toad debacle, which has caused all sorts of problems because it did not have a specific relationship with the pest that it was supposed to control.
On the other hand, chemical control will kill weeds outright, when correct herbicide rates and technique are followed and there are many things you can do to help maximize the effectiveness of chemical use.
Chemical control should be used for:
Treating priority areas to prevent weed movement to new areas, including between properties,
Removing single/isolated plants, or small patches so that they don’t turn into bigger patches,
Situations where resources are going to be available to do follow up of initial chemical treatment,
Killing weeds as quickly as possible to prevent flowering and seed drop,
Protection of priority assets.
Project Officer, Finniss Reynolds Catchment Group