Survey of Persistency of Spinosad, Imidacloprid and Indoxacarb on Trichogramma brassicae and T. evanescens

Trichogramma wasp is one of the most successful parasitic species in the world and the widespread use of these wasps has taken control of many pests in more than 30 countries. Approximately 32 million hectares of agricultural land and forests have been covered by the Trichogramma every year. This parasitoid is reared on Sitotroga cerealella (Lep.: Gelechiidae) and Ephestia kuehniella (Lep.: Pyralidae). The superiority of these bees is their ability to destroy the egg pest before its damage to the host plant. Nonselective insecticides can reduce the potential of biological control agents. In order to determine the most suitable insecticide for effective pest control of Trichogramma wasps, the present study evaluated the persistence toxicity of spinosad, indoxacarb and imidacloprid after exposing of T. brassicae and T. evanescens, the egg parasitoids of tomato leaf miner, according to IOBC procedure. The experiments were carried out in the Laboratory of Plant Protection at Agricultural and Natural Resources Research and Education Center of Markazi Province, Arak, Iran. The egg parasitoids T. brassicae and T. evanescens (Hymenoptera: Trichogrammatidae) were provided from Biological Control Department of Plant Protection Research Institute (Tehran, Iran). The parasitoids were reared on the Angoumois grain moth, Sitotroga cerealella (Lepidoptera: Gelechiidae) at the laboratory under the controlled conditions of 25±1°C, 65±10% RH and photoperiod of 16:8 h (L: D). Insecticides: The recommended doses (RD) of the insecticides of Spinosad (SC 22.8% Spintor, DowAgroscience Co.), Imidacloprid (SC35% Confidor, Bayer) and Indoxacarb (SC15% Avant, Aria Shimi) were tested.
Persistent tests: The insecticides were applied at RD on tomato plants by a hand sprayer to the point of run off. Plants were maintained under a transparent polyethylene rain cover in the field. Leaves of the treated tomato plant were sampled and transferred to the laboratory at different time intervals of 3, 5, 16 and 31 days after insecticide application. These intervals were proposed by the IOBC/WPRS Working Group. The samples were placed in ventilated plastic Petri dishes for exposure to the adult parasitoids. The experiment was repeated six times on 30 adult individuals of each species. Assessment of mortality was made on the basis of 24 h post exposure time. Persistence rate of wasps against each insecticide was evaluated according to IOBC/WPRS Working Group. The categories under laboratory conditions include: A, short lived (<5days); B, slightly persistent (5-15days); C, moderately persistent (16-30days); D, Persistent (>30 days). Persistence tests for 30% mortality at each period of time revealed that imidacloprid with less than five days persistence classified as short lived insecticides (class A) for T. brassicae and T. evanescens wasps, indoxacarb with more than five days and less than 16 days effect was slightly persistent (class B) and spinosad which persisted more than 16 days but less than 30 days was moderately persistent for the parasitoid T. brassicae and T. evanescens (class C) (Table 1, 2). This research clearly showed that imidacloprid which persisted less than five days at field conditions were compatible with both egg parasitoids, T. brassicae and T. evanescence as a part of integrated management program of pests. This study also revealed that both parasitoids responded at the same level of susceptibility after exposure to the insecticide. There is no evidence for effect of imidacloprid on Trichogramma parasitoids, however our finding was the same as the results of Fernández et al. (2017) who reported imidacloprid was safe for predatory mite Amblyseius swirskii adults. Some researchers also emphasized on computability of this insecticide with a large number of natural enemies, including the predatory mites Amblyseius cucumeris and Phytoseiulus persimilis. The insecticide was compatible with all stages of phytoseiid mites, reported by Lefebvre et al. (2011) and Beers and Schmidt (2014) in Galendromus occidentalis. Garzon et al., (2015) also reported this insecticide was nontoxic to last instar larvae and adults of Chrysoperla carnea and Adalia bipunctata so they emphasized that it is a good candidate to be incorporated into IPM programs. On the contrary, Doker et al., (2015) reported a significant reduction in oviposition of Iphiseius degenerans after exposure to imidacloprid. This research focused on the impact of insecticide exposure on Trichogramma species in Markazi Province. Pesticides that control pests without severe side effects on substantial natural enemies are always a necessity for integrated control programs. The application of the short-lived insecticides can allow a rapid recolonization of the parasitoids. We found imidacloprid is non-harmful for both T. brassicae and T. evanescence wasps. By contrast, spinosad and indoxacarb should be used with care as a part of IPM procedure.