Non-host or conditional non-host status at a particular stage of harvest maturity can be used as a phytosanitary measure to ensure freedom from fruit fly infestation. However, published records of hosts for particular fruit fly species may not always be reliable for determining non-host status for phytosanitary purposes. It is frequently difficult or impossible to validate old records. Fruit fly species may be correctly identified, but in many cases host details such as the fruit or vegetable variety, the stage of maturity, and the skin condition (damaged or undamaged) at collection were not recorded. Thus, published host records may be misleading, incomplete or incorrect for negotiating market access protocols and the development of a standard method for determining host status under defined, reproducible conditions was highly desirable. Such a method has important ramifications for international trade in many fresh fruit and vegetable commodities.
This standard uses well-known techniques in the form of a standard to provide a regular methodology for solving the problem of inaccurate host status records or the absence of host status information. These guidelines are "new" in this format and trading partners will need consultation before using them. It is likely further information will be available in the near future, so the standard will be reviewed at the next meeting of the Commission.
Where APPPC members use these guidelines, they are encouraged to inform the APPPC Executive Secretary particularly when improvements or additions are made to the techniques.
Basic guidance for host status testing includes the following:
in determining the host status of a fruit variety at a particular (described) stage of maturity, the methods outlined in this document should be adhered to;
the host status of each variety of fruit (at the described stage of maturity) should be determined separately;
each fruit fly species for which determination of host status studies are required should be tested separately;
the three stages noted in Figure 1 can be conducted sequentially or concurrently.
Figure 1: Diagram of the host testing stages
The following points should be considered as prerequisites to the commencement of host status trials:
a list of all fruit fly species occurring in the exporting country (list i)
a list of fruit fly species for which the exporting country considers host-status testing to be necessary (list ii)
information supporting the non-host status of the fruit variety concerned to some of the fruit flies found in the exporting country. Survey data should show:
that the fruit variety is not a recorded host of those fruit fly species excluded from list (ii)
that the fruit fly species excluded from list (ii) is highly host specific on one host species (ie is recorded from only one host species).
For each fruit fly species listed as requiring host-status testing, the exporting contracting party should provide the importing contracting party with reports giving the results of host-status testing in accordance with this standard.
Consultation with trading partners prior to and during trials will increase the understanding of NPPOs and their confidence in the trial results.
The standard describes tests for determining the host status of a fruit or vegetable variety at a defined stage of maturity to a particular fruit fly species (Diptera: Tephritidae). A fruit variety may be classified as a non-host, conditional non-host or a potential host on the basis of these tests.
There is a series of tests involving laboratory cages trials and field trials. Laboratory cage trials using punctured and unpunctured fruit provide a robust test and are mandatory. This system can be supplemented by field trials using punctured fruit if required. The results of trials are interpreted in the following manner:
if no survival is recorded in a damaged fruit, regardless of maturity, then the fruit is described as a non-host to the fruit fly species tested
if no survival is recorded in a damaged fruit at a specific maturity, then the fruit is described as a conditional non-host to the fruit fly species tested
if no survival is recorded in undamaged fruit at a particular stage of maturity tehn the fruit is described as a conditional non-host
if either damaged or undamaged fruit become infested, then the fruit is described as a potential host.
The term potential host is used because the trials are forced, no-choice tests using laboratory reared flies and these may over represent the population pressure actually found in the field situation.
Physical damage to fruit (i.e. breaks in the skin surface) may provide fruit flies the opportunity to oviposition where this opportunity is precluded by undamaged skin. Therefore, when determining host-status of a fruit, consideration should be given to both physically damaged and undamaged states of the fruit.
For the purpose of these trials physical damage to fruit is achieved by puncturing fruit with entomological pins. The terms punctured and unpunctured fruit are used to describe damaged and undamaged fruit in this standard, as these terms reflect the actual methods used to damage fruit in the trials.
The suggested sequence of tests is as follows:
The first test is of punctured fruit in a laboratory cage to determine if a commodity can be a host to a fruit fly species if it is punctured. The results of trials are interpreted in the following manner:
if the fruits do not become infested, then the commodity is regarded as a non-host to that fruit fly species
if the commodity does become infested, even if only one adult of that fruit fly species develops, then the commodity is either a host or conditional non-host to that fruit fly species.
The second test is a laboratory cage test using unpunctured fruit to determine if fruit may be a conditional host. The results of trials are interpreted in the following manner:
if the unpunctured fruit does not become infested, then the commodity is regarded as a conditional non-host
if the unpunctured fruit does become infested, even if only one adult fruit fly develops, then the commodity is regarded as a host unless the third test (as noted below) shows it to be a conditional non-host.
The third test is a field cage trial using unpunctured fruit to determine if a fruit found to be a host under laboratory conditions (as in the second test) may be a conditional non-host under field conditions. The laboratory cage trial are recognized as stringent tests that may not duplicate what happens in the field. The results may indicate:
if the unpunctured fruit under field conditions does not become infested, then the commodity is regarded as a conditional non-host
if the unpunctured fruit under field conditions does become infested, even if only one adult fruit fly develops, then the commodity is regarded as a host.
The number of flies that should be used in host status trials is selected to try to truly reflect field populations. This has been the subject of debate for many years. The only country that has established a standard for host testing in New Zealand. Their standard states that the number of gravid females to be used per cage should be adequate to ensure 250-500 viable eggs are laid per 500 gm of fruit. To asses the potential oviposition load that trial fruit may be exposed to, fecundity tests on colony flies are undertaken. The average fecundity per female is then used to calculate the required number of females per cage.
In this standard a minimum potential oviposition load of 1 000 viable eggs per replicate was chosen for laboratory trials. However, for field/glasshouse trials a minimum potential oviposition load of 1 500 viable eggs per replicate was chosen. The higher rate of 1 500 eggs per replicate is to compensate for higher adult mortality that may be experienced when laboratory reared flies are released in the field. Additionally, the exposure period for field/glasshouse trials is 48 hours compared to 24 hours for laboratory trials to allow laboratory reared flies to acclimatise to field conditions.
