2013 Annual Report
1a.Objectives (from AD-416):
1) Determine the effect of host larval stage, density and host to parasitoid ratio on host attack rate and critical fitness parameters of EAB larval parasitoids (Tetrastichus planipennisi and Spathius sp).
2) Determine the optimal temperature and photoperiod for the mass rearing of long-lived parasitoids capable of sustained (survival) high host-attack rates (parasitism).
3) Develop and ring-test Standard Operating Procedures (SOP) for mass rearing high quality EAB parasitoids.
1b.Approach (from AD-416):
We will use the tropical ash-based method to rear EAB larvae for tests with two different species of EAB larval parasitoids: T. planipennisi (a gregarious endoparasitoids) and Spathius sp (a newly imported gregarious, ectoparasitoid). Freshly cut tropical ash logs will be infested with fertilized EAB eggs laid on strips of un-bleached coffee filter paper. EAB egg-infested logs will be then placed into ventilated plastic boxes, and incubated under normal rearing conditions (27oC; RH 45 - 65%, and photoperiod L:D=16:8 hrs) prior to use in different tests for Objectives 1 and 2. A standardized size of the tropical ash logs (2.5 cm in diameter x 20 cm in length) will be used to rear EAB larvae for different tests.
Significant progress has been made in determining the optimal host larval stage and ambient temperature in FY 2013. Results from our study showed that host size [as measured by age class or larval stage reared at the normal temperature (25 +/- 1.5 oC)] had a significant influence on the attack rate of foraging parasitoids, with small host larvae (3.5 week-old,1st- 2nd instars) parasitized 26% of the time, and large host larvae (7 week-old, 3rd - 4th instars) parasitized 76% of the time. Small larvae (3.5week-old larvae) produced significantly fewer and more male-biased progenies than 7week-old larvae, and the anatomical measures of individual fitness (length of left hind tibia, body, and female ovipositor) were greater for individuals emerging from older larvae. Most of the larvae in the 10 week-old age class had excavated a chamber in the inner sapwood and ceased feeding (termed “J-larvae’), and were not parasitized by S. galinae in naturally infested logs. However, when these J- larvae were removed and artificially inserted into grooves made in the host plant, they were parasitized by S. galinae and progeny was produced. In contrast, S. galinae never attack artificially inserted pre-pupae or pupae, which completely cease feeding activities. Those findings showed that actively feeding late instar larvae (about 7 weeks old), but not J-larvae, pre-pupae or pupae, should be used to rear S. galinae with naturally-infested host plants most efficiently in the laboratory. When artificially inserted into the host plant, however, J-larvae may still be suitable to S. galinae production.
As for optimal ambient temperature for rearing S. galinae, we discovered that foraging S. galinae parasitized emerald ash borer larvae (3rd – 4th instars) at the temperature range from 15 C to 30 C, but not at 35 C. While the developmental time for S. galinae eggs and larvae appeared to be significantly shortened from >12 weeks at 15 C to about 3 weeks at 30 C, S. galinae eggs never hatched at 35 C. Together, these results indicate that the optimal temperature range for successfully raring S. galinae is between 15 – 30 C with shortened life cycle and increased population growth rate as the temperature increases within this range.