Host plant water deficit stress impairs reproduction and development of the galling fly (Parafreutreta regalis), a biological control agent of Cape-ivy (Delairea odorata)

Authors: Portman, Scott; SANTACRUZ, KYRA - University Of California; Moran, Patrick

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2021
Publication Date: 2/17/2021
Citation: Portman, S.L., Santacruz, K., Moran, P.J. 2021. Host plant water deficit stress impairs reproduction and development of the galling fly (Parafreutreta regalis), a biological control agent of Cape-ivy (Delairea odorata). Biological Control. 156. Article 104555. https://doi.org/10.1016/j.biocontrol.2021.104555.
DOI: https://doi.org/10.1016/j.biocontrol.2021.104555

Interpretive Summary: The non-native weed known as Cape-ivy, a vine-like plant in the sunflower family, is one of the worst invaders near streams and in bluffs and forests along the California coast, smothering native plants and clogging streams in these sensitive habitats. Chemical and mechanical control and hand-pulling cannot keep up with the rapid growth of this plant, and cannot reach Cape-ivy stems in remote areas with rugged terrain. The USDA-Agricultural Research Service (ARS) discovered a fly in the native range of Cape ivy in South Africa, known scientifically as Parafreutreta regalis, that is a potential biological control agent. Adult female flies lay eggs inside Cape-ivy shoot tips, turning them into tumors or ‘galls’. Larvae feed inside on the gall tissue, chew holes in the gall, pupate inside, and emerge after about two months to begin the life cycle again. In 2016, ARS received a permit to release the fly in California, and releases have been made at 19 sites. Drought is a common climatic phenomenon in California, which has a Mediterranean climate with almost no rain between April and October. Cape-ivy stems that are not close to a water source often wilt during the daytime in the summer and fall, partially recovering at night. ARS researchers simulated chronic drought in a greenhouse by giving potted Cape-ivy plants less water. The treatment worked, as the plants exposed to drought grew 50% more slowly and had up to 66% reduced weight. Adult females offered a choice in a cage between drought-stressed and normally- watering plants produced more galls on normally-watered plants, showing that they prefer well-watered plants. In separate ‘no-choice’ tests, females were given only one plant, exposed to drought stress just for five days, while she was laying eggs; during the two-month period after parent females were removed, when galls were developing; or for the entire fly life cycle. In these no-choice tests, chronic drought (during the gall development phase or the entire life cycle) reduced the number and size of galls (by up to 51% and 70%, respectively), leading to the production of smaller pupae inside galls. Chronic drought increased total development time (egg to adult) by 15% and led to up to 80% fewer adults emerging from galls. The results showed that drought can reduce the ability of the Cape-ivy fly to establish large populations for biological control of Cape-ivy in California. Field releases of the fly should avoid sites with chronic drought.

Technical Abstract: Drought leading to water deficit stress is known to reduce performance of galling insects. The shoot tip-galling fly Parafreutreta regalis has been released for biological control of Cape-ivy (Delairea odorata) in California. Lack of moisture during the dry season commonly causes wilting of Cape-ivy shoots, which is a concern because the quality of the plant could influence the fly’s ability to establish and multiply. We imposed water deficit stress on potted Cape-ivy plants, then measured the plant’s and insect’s response to water deficit compared to fully-watered plants. Water deficit stress was imposed during female oviposition (short-term), gall development (long-term), or full insect life cycle (long-term). Plants subjected to long term water deficit stress showed declines in shoot growth, leaf area, and total biomass. In choice tests, where mated female flies were provided both water deficit stressed and non-stressed plants, greater numbers of galls were found on non-stressed plants, but there were no differences in the numbers of insects per gall (ie eggs laid). In no-choice tests, long term water deficit reduced the number and size of galls (by up to 51% and 70%, respectively), leading to the production of smaller puparia, increased total development time (egg to adult) by 15%, and led to up to 80% fewer adults emerging from galls. Our study shows that P. regalis will oviposit and develop successfully on water deficit-stressed plants, but declines in female preference, insect development and abundance could inhibit fly establishment and/or population growth under drought conditions.