Aster leafhopper survival and reproduction, and Aster yellows transmission under static and fluctuating temperatures, using ddPCR for phytoplasma quantification.

Aster yellows (AY) is an important disease of Brassica crops and is caused by Candidatus Phytoplasma asteris and transmitted by the insect vector, Aster leafhopper (Macrosteles quadrilineatus). Phytoplasma-infected Aster leafhoppers were incubated at various constant and fluctuating temperatures ranging from 0 to 35 °C with the reproductive host plant barley (Hordium vulgare). At 0 °C, leafhopper adults survived for 18 days, but failed to reproduce, whereas at 35 °C insects died within 18 days, but successfully reproduced before dying. Temperature fluctuation increased thermal tolerance in leafhoppers at 25 °C and increased fecundity of leafhoppers at 5 and 20 °C. Leafhopper adults successfully infected and produced AY-symptoms in canola plants after incubating for 18 days at 0-20 °C on barley, indicating that AY-phytoplasma maintains its virulence in this temperature range. The presence and number of AY-phytoplasma in insects and plants were confirmed by droplet digital PCR (ddPCR) quantification. The number of phytoplasma in leafhoppers increased over time, but did not differ among temperatures. The temperatures associated with a typical crop growing season on the Canadian Prairies will not limit the spread of AY disease by their predominant insect vector. Also, ddPCR quantification is a useful tool for early detection and accurate quantification of phytoplasma in plants and insects.

Constant versus fluctuating temperatures in the interactions between Plutella xylostella (Lepidoptera: Plutellidae) and its larval parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae).

Laboratory studies were conducted to determine the effects of constant temperatures (7, 22, and 30°C) and corresponding fluctuating temperatures (0-14, 15-29, and 23-37°C) on the development of diamondback moth, Plutella xylostella (L.), and its North American parasitoid Diadegma insulare (Hellén). Parasitized third-instar diamondback moth larvae were reared until adult mortality in individual thermal gradient cells at different temperature regimes. Larval mortality, parasitism success, pupal mortality, larval and pupal developmental time, adult longevity, and pupal and adult dry weight were recorded. Overall diamondback moth larval mortality was low. The pupal mortality of D. insulare increased with increasing temperature; however, diamondback moth did not show such a response. Greatest parasitism success (67%) was found at constant and fluctuating 22°C and fluctuating 7°C, and the lowest (30%) at fluctuating 30°C. Longer development times and greater pupal body masses occurred at lower temperatures for both insects. Significant differences occurred between constant and fluctuating temperature regimes for most parameters of both insects. Fluctuating compared with constant temperatures caused shorter development times, similar body mass, and higher adult longevity for both insects at optimal and lower temperature ranges. Both insects experienced 0°C at fluctuating 7°C (0-14°C) and survived. These results have important implications for extrapolating temperature effects on insects in laboratory studies with constant temperatures. Comparing successful parasitism capacity of the wasp and pupal survival and body mass of both host and parasitoid, we conclude that D. insulare is a more effective parasitoid at lower temperatures.