First step would be to engage with a pest control provider.  Try and give them as much detail as possible (photo's, country of origin etc).  They may then be able to advise and possibly deal with the immediate issue.

Second step review your risk assessment with regards to cargo handling and container opening and include details of any findings from the pest controller, who may advise on what creatures you are dealing with - only then can you determine appropriate control measures.  Also including the fumigation exposure.

Third step - arrange for your supply chain to ensure better protection against unwanted pest becoming inadvertantly shipped within the containers.

Just a few immediate thoughts of the top of my head.....  Not sure but there may be a requirement (depending on what species etc. are found) for reporting this to your local authority or similar (environment agency??) for non-native species being found.