Evaluation of Orius species for biological control of Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae)

Key words: Thysanoptera, Frankliniella occidentalis, Heteroptera, Orius leavigatu, Orius majusculu, Orius niger, Orius insidiosus, Biology, Diapause, Biological control.

The overall aim of this research was to develop a biological control programme for F. occidentalis through the selection of an efficient beneficial arthropod. First, a general review of the literature about thrips pest species in Europe and in particular of Frankliniella occidentalis (Pergande) (Western Flower Thrips) was made. Information regarding the biology, distribution, host plants of thrips and damage induced by this pest species were discussed and summarized. The main candidates as natural enemies for control of thrips emerging from this literature study and from an evaluation of all present data, were Anthocoridae, and, thus, further research was directed towards Anthocorid predators of the genus Orius (Rhyncota: Heteroptera) ( chapter 1 ).

Next, of the genus Orius , the most common species of the Mediterranean regions of Europe were chosen as candidates for biological control of F. occidentalis . Orius predators were collected in several areas in Italy on 36 plant species infested by thrips. The most common species were O. niger Wolff, O. laevigatus (Fieber) and O. majusculus (Reuter). No clear host-plant preferences of these Orius species were recorded ( chapter 2 ).

Consequently, biological characteristics and predation activity of four Orius species (the palaeartic O. majusculus , O. laevigatus and O. niger and the neartic O. insidiosus , an exotic species that was earlier released in Italy) were determined by laboratory experiments using two prey species: Ephestia kuehniella (Zell.) eggs and Frankliniella occidentalis adults. Preimaginal mortality, development time, sex-ratio, pre-oviposition period, longevity, fecundity, and predation during the instar stages and the adult stage were measured. The intrinsic rates of natural increase (r _m) and the kill rates (k _m= ln k ₀/t _k) for all four Orius species was determined. The k _mwas 0.23 for O. laevigatus , 0.21 for O. majusculus , 0.25 for O. insidiosus , 0.19 for O. niger , respectively. In all species, the females that fed on E. kuehniella showed greater longevity and higher reproduction than those fed on F. occidentalis . Most data for the neartic O. insidiosus were similar to those of O. laevigatus and O. majusculus . Mass rearings of O. insidiosus , O. laevigatus and O. majusculus were successfully developed, while O. niger appeared difficult to rear. Based on these data, it was concluded that O. laevigatus might be the best candidate for control of thrips ( chapter 3 ).

No data were available about the occurrence of diapause in O. laevigatus . As thrips pest occur early in the season, it is important to use natural enemies that do not go into diapause. The possibility of inducing a reproductive diapause in this palearctic species was therefore investigated in the laboratory using two strains: strain N collected in northern Italy (Po Valley) and strain S collected in southern Italy (Sicily). The influence of photoperiod on Orius eggs was studied. Development time, adult emergence, sex ratio, pre-oviposition period, fecundity, and the presence of mature oocytes were recorded. The two strains of O. laevigatus showed to have a different way of overwintering: in the northern strain part of the population undergoes a weak reproductive diapause, while for the southern strain overwintering could best be described as quiescence ( chapter 4 ).

Finally, the capacity of O. laevigatus to control thrips pests ( F. occidentalis and T. tabaci ) was studied by releases of this predator in two vegetable crops in commercial greenhouses, sweet pepper and eggplant. The releases of the pirate bugs were made as soon as thrips were detected, resulted in early establishment of the predator, in an interaction between prey and predator at low population densities and often in sufficient control of the pest ( chapter 5 and 6 ).

In conclusion, the southern Italian strain of O. laevigatus showed to be an efficient natural enemy of thrips and F. occidentalis . This natural enemy is currently produced and commercially used on large scale in Europe to control thrips species in vegetable and ornamental crops, mostly in protected crops ( chapter 7 ).