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Organisms often live in unpredictable environments and have to adopt life history strategies that optimize their fitness under these conditions.According to bet-hedging theory,individuals can reduce variation in fitness outcomes by investing in different strategies at the same time.For arthropods,facultative summer diapause enables survival during dry and hot periods of the year,and can be triggered by a decrease in resource abundance.However,the effect of resource depletion on diapause induction has never been disentangled from the effect of the perception of the presence of competitors.Using two solitary parasitoid species of cereal aphids as a model system,Aphidius avenae (Haliday) and Aphidius rhopalosiphi (De Stefani-Perez) (Hymenoptera:Braconidae),we tested whether (i) low absolute host density and/or (ii) high levels of parasitoid females’ competition lead to matal-induced summer diapause in parasitoid offspring.Under summer-like climatic conditions,emerging parasitoid females were (i) reared alone and exposed to different host densities (from 5 to 130 aphids),or (ii) reared together with competing females (from 2 to 20 females) and then exposed individually to 50 aphids.For both parasitoid species,low aphid densities did not induce summer diapause.However,the incidence of summer diapause increased up to a maximum of 11％ with increasing levels of competition experienced by female parasitoids.More than 60％ of the females produced both diapausing and nondiapausing offspring after being kept at the two highest competition densities.Such a spreading-the-risk strategy has likely evolved to optimize parasitoid fitness by preventing the following generation from exposure to low populations of suitable hosts and high mortality from superparasitism.These results provide the first experimental evidence of direct matal competition-induced diapause in insects,and may change the way we apprehend the evolution of arthropod seasonal ecology,by considering intraspecific competition.