Bollettino dell Istituto di Entomologia (Guido Grandi) (2000) 54, 1-11

Fabrizio Santi and Bettina Maccagnani (2000)
Influence of the humidity on mortality rate and embryonic development time of two strains of Phytoseiulus persimilis Athias-Henriot (Acarina Phytoseiidae)
Bollettino dell Istituto di Entomologia (Guido Grandi) 54, 1-11
Abstract: The effect of relative humidity on egg hatching and its impact on embryonic development time were studied. Two strains of different geographic origins were compared: the SI strain, collected in Sicily, and the BU strain, supplied by the Ciba - Bunting biofactory. To produce the eggs necessary for the experiments, the two strains were reared in climatized cells, placed in separate rooms. The temperatures and the relative humidity chosen for the experiments were obtained through the method of the sature saline solutions in isolated environment. Several experimental units, with all the different RH needed, were prepared.
To study the influence of RH on egg hatching, groups of eggs of the two strains were exposed to the following constant relative humidity until they either hatched or collapsed: 50±2%, 73±2%, 90±2%. The effect of the exposition to a variable RH was also tested as follows: 3 hours at 90% RH, 16 hours at 50%, then 8 hours at 90% RH, 16 hours at 50% RH and then at 90% RH, till the last control. To study the effect of prolonged period of stress at low RH on the embryonic development time, for each P. persimilis strain, groups of eggs were exposed to increasing periods of stress at 50±2% RH, 24, 28, 32, 36 hour. A fifth group of eggs was continuously maintained at the optimal RH (90±5% RH) thus acting as control. All trials were run at the constant temperature of 26±1°C, which is the optimum one for rearing the beneficial mite.
Relative humidity confirmed to be crucial for P. persimilis development, which need, for a full success in egg hatching, constantly a very high humidity rate (90%). As the humidity decreases, the percentage of hatched eggs significantly decreases, and is reduced to zero with RH around 50%. Trials run within a variable humidity range evidenced a good performance of the BU strain, whose percentage of eggs hatching was not different from that obtained in the optimal conditions.
This allowed us to suppose that the duration of the exposition at the low humidity stress is the limiting factor. In fact, only long expositions (more than 28 hours) at 50% RH cause a significant reduction in the percentage of eggs hatching, as for an exposition inferior to 28 hours the percentage of hatched eggs was higher than 70%. The low humidity level has also a strong impact on the duration of the embryonic development time, as both strains took nearly one day more to complete the embryonic development, with respect to the control group, if exposed for 32 hours to 50% RH.
In general we can say that the BU strain registered a higher percentage of hatched eggs with respect to the SI strain, showing a better resistance to insufficient or not optimal humidity level, even if these differences could be reduced till disappearing along with the mass rearing. Anyway, the SI strain, in other researches, obtained better results when other important parameters were considered, such as female fecundity and longevity. A precise examination of many different biological parameters is needed for the identification of the more suitable strain for every particular environment.
Finally, we suggest that since the methodology applied in this study is sufficiently simple and very precise, it can be adopted as a specific test in quality control programs.
(The abstract is excluded from the Creative Commons licence and has been copied with permission by the publisher.)
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Database assignments for author(s): Fabrizio Santi

Research topic(s) for pests/diseases/weeds:
biocontrol - natural enemies
Research topic(s) for beneficials or antagonists:
environment/habitat manipulation

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
Phytoseiulus persimilis (predator)			Italy