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United States Department of Agriculture

Agricultural Research Service


Location: Beneficial Insects Introduction Research

2013 Annual Report

1a.Objectives (from AD-416):
Objective 1: Determine the effect of host larval stage, host density and host to parasitoid ratio, on host attack rate and critical fitness parameters of EAB larval parasitoids (Spathius sp and/or Tetrastichus planipennisi). Objective 2: Determine the optimal temperature and photoperiod for host-attack rate (parasitism), and parasitoid progeny survival and development. Objective 3: Develop and ring-test standard rearing protocol 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 EAB larval parasitoids: Spathius sp (a newly imported gregarious, ectoparasitoid) and/or T. planipennisi (a gregarious endoparasitoids). 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. Optimal host larval stage, density, host to parasitoid ratio, temperature and photoperiod for the production of the fittest and most fecund parasitoids will be determined independently in a series of laboratory assays.

3.Progress Report:

Following the recruitment of a graduate student assistant, a series of experiments have been conducted to determine the effect of host larval stage, density and host to parasitoid ratio on EAB parasitoid (Spathius galinae) progeny productionve 1). Assays have been performed with four larval instars of emerald ash borer reared with our new tropical ash egg-infestation method. Several assays were also performed using inserted mature larvae (J-larvae), prepupae, and pupae collected from wild populations. Results from our study showed that host size [as measured by age class or larval stage reared at the normal temperature (25 +/- 1.5 degrees C)] 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 attacked artificially inserted pre-pupae or pupae, which completely cease feeding activities. Those findings showed that actively feeding late instar larvae (~7week 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, 12 replicates of pairs of S. galinae were tested at each of the five constant ambient temperatures (15, 20, 25, 30, and 35 C) for their survivorship, weekly oviposition rate, and life-time realized fecundity. Results from the experiment showed 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.

Last Modified: 8/25/2016
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