The effect of Beauveria bassiana on the host location of rice striped stem borer, Chilo suppressalis by Trichogramma japonicum

The rice stem borer, Chilo suppressalis Walker (Lepidoptera: Crambidae), is a key rice pest, causing major yield loss in Asia. Trichogramma japonicum Ashmead (egg parasitoid) and Beauveria bassiana Balsamo Vuillemin (entomopathogenic fungi) are two important organisms used in sequence or integrated for C. suppressalis management. However, few information is available on potential antagonism between the two biocontrol agents. Under laboratory conditions, we assessed whether the presence of B. bassiana had an impact on the oviposition preference of T. japonicum for C. suppressalis eggs. We characterized parasitic behavior of T. japonicum exposed or not to B. bassiana on C. suppressalis eggs (varying in age) by carrying out observation experiments and using a Y-tube olfactometer. We also evaluated the response of T. japonicum to B. bassiana-affected vs. -unaffected C. suppressalis eggs. Olfactometer tests showed that T. japonicum-vectored B. bassiana and/or C. suppressalis eggs immersed in B. bassiana did not have major effects on choices made by the parasitoid. T. japonicum attacked C. suppressalis eggs of 0-d, 2-d and 4-d old and had preference to fresh host eggs (0-d). However, the contaminated T. japonicum were not able to distinguish between eggs of 0-d old and 4-d old in contrast with the uncontaminated parasitoids. A reduction in insect locomotion (no-response number) of T. japonicum contaminated by B. bassiana preferring to 4-d-old host eggs was found, which was compared to the no-response number of uncontaminated T. japonicum preferring to 0-d-old host eggs. The parasitic behavior of host searching time, checking time, drilling and oviposition time and the parasitism rate (in 24 h) did not differ between the fresh eggs immersed with B. bassiana and control (eggs without B. bassiana). On the other hand, searching and checking times were slightly higher when parasitoids were contaminated by B. bassiana (drilling and oviposition times as well as the parasitism rate in 24 h were not impacted). Therefore, T. japonicum may be used together with B. bassiana in integrated program targeting C. suppressalis.