Probit mortality

Converting mortality data to ‘probits’ allows calculation of the dose required to kill a certain percentage (eg 50%) of the population. Postharvest quarantine treatments are often required to provide probit 9, equivalent to 99.9968% mortality. Given the low numbers of live pests likely to be present in any one consignment, this represents an extremely high level of quarantine security. 

The standard often required of postharvest quarantine treatment is ‘Probit 9’. Probit analysis was developed to linearise the sigmoid (S-shaped) dose response curve typical of insect mortality. This allows calculation of percentage mortality at different doses. For example, LD50 (the dose that kills 50% of the population) can be accurately calculated using this type of analysis.

Probits are also used to provide confidence in the result, as they are based on the number of insects killed. Killing a single insect provides only a very low level of confidence that the method will be effective, even if calculated mortality is 100%. Killing 1,000 insects provides considerably more confidence in the method, and killing 100,000 from a population of 100,000 provides a great deal of confidence.

Probit 9 mortality was first proposed as a measure of quarantine certainty by Baker (1939) in his paper ‘The basis for treatment of products where fruit flies are involved as a condition of entry to the United States’. Probit 9 is equivalent to a mortality of 99.9968%. In effect, this represents three survivors from an initial population of 100,000 insects.

Mortality of fruit fly exposed to a cold treatment, expressed as percentage mortality over time, and with the data converted to Probits. 

There are many reasons Probit 9 is not necessarily the best method of developing a quarantine treatment. Criticisms of Probit 9 include: 

  • It does not take into account the actual risk involved, that is, the probable infestation rate in the consignment 
  • It is an inappropriate use of the Probit analysis method 
  • It is an arbitrary standard not based on scientific method 

 Despite this, proof of either Probit 9 or Probit 8.7 mortality is still required by many importing countries. To standardise how this is achieved, the Japanese Ministry of Agriculture and Fisheries set out experimental procedures for demonstrating acceptable mortality. These measures have now been broadly adopted by many countries: 

  • A series of initial tests (minimum three full replications) are conducted to determine the most treatment-tolerant life stage of the target insect species (eg first, second or third instar larvae) 
  • These are followed by three large-scale trials under semi-commercial conditions (eg minimum loading rates in a cool room may be stipulated) 
  • Each large-scale trial should apply the proposed treatment to at least 10,000 larvae of the most tolerant treatment stage 
  • The treatment is regarded as successful if there are no survivors.