Environmental Effects on Polyphagous Shot Hole Borer
ABSTRACT OF THE DISSERTATION
Effects of Pharmaceuticals on Insects with Different Feeding Guilds
Colin Yoneji Umeda
Doctor of Philosophy, Graduate Program in Entomology
University of California, Riverside, September 2017
Professor Timothy Paine, Chairperson
Polyphagous Shot Hole Borer (Euwallacea fornicatus) (PSHB) is an ambrosia beetle that has invaded southern California and poses a significant threat to many species of trees, both introduced and native. It is associated with three different fungal symbionts which it grows on the walls of the galleries it forms on the inside of its tree hosts. Through a combination of mechanical damage and fungal invasion, PSHB causes dieback and eventual death of its host tree. Very little is known about PSHB as it was originally thought to be a different, congeneric beetle species. The overall goal of this dissertation is to expand the body of knowledge on the life history characteristics of PSHB, especially with regards to development. This research helps define the environmental conditions that are vital to PSHB development and estimates potential areas of risk.
PSHB reared under different temperature conditions exhibited similar developmental responses to its congener the Tea Shot Hole Borer. Its minimum and maximum temperature thresholds were around 13°C and 33°C, respectively. PSHB development rate was calculated to be optimal near 28 °C.
Using the temperature requirements of PSHB it was possible to construct a climate model with MaxEnt and CLIMEX. Using these programs, it is shown that PSHB can spread throughout the southern portion of North America into South America. With respect to California, the climate is suitable enough to allow PSHB to spread all the way through the central valley to northern California.
When comparing attack rates on host trees with different watering regimes, in three different studies, it was found that irrigation did not impact the rate of attack. In these studies, irrigation only affected the level of crown senescence.
When examining the individual effects of PSHB fungal symbionts it was found that only two of the three, Fusarium euwallaceae and Graphium euwallaceae, were able to provide enough nutrition on their own to allow for larval development to adulthood. Larvae raised on F. euwallaceae were larger than those raised on G. euwallaceae, but developed slower near optimal temperatures which could indicate a trade-off.