Survey and Rearing of Grape Berry Moth Larval Parasitoids in Northwest Pennsylvania

In this study, we will Identify larval parasitoids of GBM from samples collected in wild and cultivated grapevines in Northwestern PA. These parasitoids will be reared in laboratory conditions to determine their predatory capacity, the target GBM life stage they prey on, and their life cycle. Afterwards, we will choose at least one parasitoid species for mass rearing and release back into the field. Below are the procedures for each proposed objective.

Methods for objective 1.  “To identify larval parasitoids of grape berry moth in Northwestern Pennsylvania.” During the first year of the project (growing season of 2022), we will sample GBM larvae in six sites with historically high GBM infestations. These vineyards are located in the Erie county of Northwest PA and have been monitored for several years by our extension specialist Andrew Muza. Four of the six sampling sites are located in North East, PA (east to Erie city), whereas the remaining two sites are located west of Erie city; the location of each site is illustrated in Figure 2. Samples will be collected every two weeks from commercial vineyards and wild grapes growing adjacent to cultivated plots. The samplings will start one week after the wild grapes bloom (~ first week of June) and continue throughout the growing season. We will make an effort to sample at least 100 GBM-infested grapes per sampling site. These samples will be taken to the laboratory at the Penn State Lake Erie Regional Grape Research and Extension Center (LERGREC) located in North East, PA (662 N. Cemetery Rd, North East, PA16428); the samples will be placed in plastic containers with a fine screen mesh and will be incubated at 25C with a photoperiod of 15 h day and 9 h dark. The samples will be inspected daily for the emergence of larval parasitoids. Additionally, GBM pupa will be thoroughly inspected for parasitoids that may not emerge until the GBM larvae transitions to the pupal stage. We will keep record of the number of parasitoids recovered per number of GBM-infested grapes to calculate the percentage of parasitism per species. We will also keep record of the time at which the parasitoids emerge and the GBM-developmental stage. Some of the emerged parasitoid adults will be preserved in ethanol for further taxonomic identification, and the rest will be used to start colonies in the lab.

Methods for objective 2. “To create a rearing protocol for at least one of the parasitic species identified.” Emerged adult parasitoids will be transferred to pop-up cages containing diluted honey and water in cotton balls as a food source; the cage will also contain living GBM stages for oviposition. GBM larvae will be provided daily and retrieved 24 h later during the lifetime of the adult parasitoids. We will keep record of the number of parasitoids reared and their developmental time. Parasitoids will be reared in incubators at LERGREC at 25C with 15 h day/ 9 h dark photoperiod. GBM colonies for parasitoid rearing will be maintained using table grapes purchased at grocery stores for food and oviposition; the insects will be kept in plastic containers with a fine breathing mesh at 25C with 15 h day/ 9 h dark photoperiod.

Methods for objective 3. “To test the efficacy of at least one parasitic species in reducing grape berry moth infestations in a commercial vineyard plot.” During the second year of the project (growing season of 2023), we will release at least one of the laboratory-reared parasitoid species at the wood edges of a commercial vineyard plot with historically high GBM infestation. We expect to do at least one parasitoid release early in the growing season (June). The parasitoid release needs to be synchronized with the presence of susceptible GBM stages in the field. For instance, if the parasitoids oviposit on recently hatched GBM larvae, we need to make sure they are released shortly after GBM oviposition. To find the appropriate time for parasitoid release, we will use the GBM degree-day model to predict the developmental stage of this insect in the field (Tobin et al., 2003). Additionally, we will randomly sample GBM-infested grapes and dissect them in the lab to confirm the occurrence of the appropriate GBM life stage before parasitoid release. In the subsequent weeks, we will do periodic evaluations (every two weeks) of GBM infestation by visual inspection of at least 50 randomly selected grape clusters within the first five rows of the vineyard edges.

Because natural GBM parasitism does occur in the field, we will also measure GBM infestations from a “control” vineyard plot with historically high insect pressure in which parasitoids will not be released. Both vineyard plots will be free of insecticide applications during the time of the experiment.

Differences in the percentage of parasitism will be calculated between control vineyard plots and those with parasitoids released using a Chi-squared test at a significant level of alpha = 0.05.  

Expected accomplishments by the end of the project. By the end of this project, we expect to have a list of GBM larval parasitoid species recovered from the field, a rearing protocol for at least one of the parasitoid species, and at least one field trial of the efficacy of one parasitic species in reducing grape berry moth infestations in a commercial vineyard plot. However, testing the efficacy of parasitoids in field conditions will depend on our ability to rear a large number of the given species in laboratory conditions. In case this cannot be accomplished, the project will still provide relevant information to the composition of natural enemies of GBM in northwestern PA that could potentially be used in sustainable pest management strategies.

Year 2022

During 2022 we advanced towards completion of our Objective 1 “To identify larval parasitoids of grape berry moth in Northwestern Pennsylvania.”  We sampled GBM larvae in two sites with historically high GBM infestations located in the Erie County of Northwest PA. The sampling took place in June and July of 2022. GBM-infested grapes were taken to the laboratory at the Penn State Lake Erie Regional Grape Research and Extension Center (LERGREC) located in North East, PA (662 N. Cemetery Rd, North East, PA16428); the samples were placed in plastic containers with a fine screen mesh and were incubated at 25C with a photoperiod of 15 h day and 9 h dark. The samples were inspected daily for the emergence of larval parasitoids. Additionally, GBM pupa were inspected for parasitoids. We recorded the number of parasitoids recovered per number of GBM-infested grapes to calculate the percentage of parasitism per species. The emerged parasitoid adults were preserved in ethanol for further taxonomic identification.

We were able to collect three different GBM larval parasitoid species Glypta mutica & Enytus obliteratus (Hymenoptera: Ichneumonidae), and Bracon scrutalor (Hymenoptera: Braconidae). Parasitism was higher early in the season in samples collected from wild grapes (Table 1). During 2023, we will include more sample sites and possibly other grape cultivars besides Concord to see if we can identify more species.

Advance towards completion of objective 2. “To create a rearing protocol for at least one of the parasitic species identified.” Emerged parasitoids of Glypta mutica (Hymenoptera: Ichneumonidae) were transferred to pop-up cages containing diluted honey and water in cotton balls as a food source; the cage also contained living GBM eggs and first instar stages for oviposition. We were able to rear 8 parasitic wasps, we could have reared more but we didn’t have enough GBM eggs to rear these wasps. Efforts are now focusing on improving our GBM lab colony to rear more parasitoids in Spring of 2023.

Table 1. Collection of GBM-infested berries for parasitoid identification

2023 Progress Report

Progress towards objective 1: To identify larval parasitoids of grape berry moth in northwest Pennsylvania. 

We sampled grape berry moth (GBM) infested grapes from six vineyards during the grape growing season of 2023 (Jun 15-Sep 4). The first sampling was conducted in wild grapes located at the edges of the vineyards, whereas the subsequent samplings were done in commercial vineyards of Concord grapes. The highest parasitism was observed in the first sampling of the season in wild grapes, however the number of GBM stages found were very few. The second highest parasitism rate was found in the sampling of Aug 7 with an average of 16.4% across sites while the lowest was the sampling of July 11 with 2.7%. Site 1 had the highest parasitism across sampling dates with an average of 21.7% followed by site 6 with 13.6% and site 2 with 12%. Site 3 had the lowest parasitism rates with an average of 3.4% (Table1). During the growing season of 2023, we collected at least five different GBM parasitoid species, currently classified to the genus level, except for three different genera presented as Ichneumonidae (Table 2). From these, the most abundant were Bracon sp1 representing 56.6% and Cydomusa sp1 accounting for 18.6% of all parasitoids found (n = 113). The least abundant parasitoid was Bracon sp 2 with only one specimen collected (0.9%). Cydomusa sp1, and sp2 were found from June to August, whereas Bracon sp 1 was found from late July to early September (Table 2). Bracon sp1 and Goniozus are ectoparasitoids and the others are endoparasitoids of GBM larvae. The taxonomic identifications reported during this year are more accurate than the ones previously reported.

Table 1. Total GBM larval parasitism in field conditions per sampling site throughout the 2023 growing season.