Estimating sample size for four methods used to recover Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae), an egg parasitoid of emerald ash borer

Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae) is an egg parasitoid of emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae).  Recovery of O. agrili can be difficult because of its small size and the cryptic placement EAB eggs in bark cracks and crevices. During 2016–2018, we compared four O. agrili recovery methods: 1) rearing adults from bark (bark rearing); 2) sifting parasitized eggs from bark (bark sifting); 3) placing sentinel EAB eggs in screened envelopes on ash trees (sentinel eggs); and, 4) placing yellow pan traps on ash trees to capture adult parasitoids.  Yellow pan traps and bark sifting had the highest mean O. agrili recoveries and percent O. agrili positive trees for all years.  In 2016, we sampled 40 contiguous ash trees within 0.25-ha-plots at each of 4 sites where O. agrili is established in Michigan with each recovery method.  We bootstrapped sample sizes of 2–40 trees to estimate the probability of recovery and sampling precision as a function of sample size.  We used a nonlinear function to predict the number of sample trees needed to have 95% probability of recovering O. agrili if it is established.  We found that sampling 10 trees that had recent evidence of EAB attack (i.e., fresh woodpecker feeding holes) with yellow pan traps or bark sifting provided >95% recovery of O. agrili if it was established and maximized sampling precision benefit: cost ratio.  Much higher sample sizes are needed to reach 95% recovery for sentinel eggs and bark rearing.