2020-21 Rapid Ag: Innovations in Controlling Riceworm in Cultivated Wild Rice
Agronomy and Plant Genetics
Riceworm is the most problematic pest in cultivated northern wild rice resulting in severe economic losses if insecticides are not applied in a timely manner. In 2018, the European Union (EU) implemented new regulations capping the maximum residue levels (MRL) of lambda-cyhalothrin, the active ingredient in the most widespread riceworm control utilized by the cultivated wild rice industry. As a high percentage of Minnesotan cultivated wild rice is exported to the EU (~70-85%), there is an urgent need to evaluate alternative insecticides for the management of riceworm. We propose to evaluate five different modes of action for riceworm management at two grower locations (Gonvick, MN and Aitken, MN) during the 2019 and 2020 growing seasons. Due to the unique challenges of working in an aquatic agricultural production system, we also propose to evaluate the feasibility of applying insecticides in the research program via drone. This will improve the safety of pesticide applicators in this challenging environment and if successful, can be used for other pesticide applications. Additionally, we propose to use water sensitive paper to compare research application techniques to best replicate expected results obtained from aerial applications, the standard commercial production practice.
Northern Wild Rice (WR) (Zizania palustris) is an important but low acreage crop in Minnesota and has historic cultural ties to the state; it is a major traditional food source and sacred part of the culture of Minnesota’s Native American peoples and is the official state grain. Wild rice has been commercially produced in Minnesota for approximately 70 years and is now considered a gourmet food item in the U.S. Despite limited acreage, Minnesota is the leading producer of WR in the U.S., harvesting 867,471 CWT in 2012 representing a contribution to the state’s economy of over US$2M (USDA-NASS 2018).
A number of insects feed on WR, by far the most problematic of which is the riceworm (Apamea apamiformis [Guenee]) and, to a much lesser extent, the Rice Stalk Borer (Chilo plejadellus Zincken), both larvae of N. American moth species. Occasional, but not consistent, losses have been the result of Rice stalk borers (Chilo plejadellus Zincken), rice water weevils (Lissorhoptrus spp.), rice leafminer (Hydrellia spp.), and rice stem maggot (Eribolus longulus Loew) (Oelke et al. 1999). Riceworm attacks native wild rice but infestation rates increase with increased biomass and N sediment content (Dahlberg & Pastor 2018, Sims et al. 2012a&b), both conditions consistent with commercial fields. A host shift to commercial wild rice and subsequent increases in native populations of Riceworm was, in view of this data, not surprising.
The native status of Riceworm’s and Rice Stalk Borer means their life cycles and seasonal development are closely synchronized with WR, meaning infestations are problematic (Oekle, 1993). Adults of both insects enter wild rice fields in late June / early July during WR flowering and lay eggs in the flowers over a period of days to several weeks. The eggs of Riceworm are laid in the wild rice flowers and after hatching the larvae begin to feed on glumes in the spikelet and then directly on kernels (MacKay & Rockburn 1958). After hatching, the larvae of Rice Stalk Borer burrow into the plant’s stem and tunnel down the stem to just above the water line where they feed until fall (Peterson et al. 1981).
Both species are native, indeed regional, and so there are few additional natural mortality factors (e.g. predators, disease) that can be used to augment control of Riceworm populations; all native mortality factors are already present and at play, suppressing populations from year-toyear. When these mortality factors decrease are adversely affected (e.g. by environmental conditions), Riceworm populations can reach outbreak status. The same is true for Rice Stalk Borer and other many other insect pests that may infest WR. Consequently, the only reliable method of suppressing Riceworm populations in WR is the application of insecticides.
As a small commodity with a unique aquatic production system, pest control in cultivated WR can be challenging. The registration of new pesticides for minor crops does not yield high revenue for manufacturers and therefore, alternative routes, including IR-4 or pesticide emergency exemptions, must be pursued. In 2018, the European Union implemented new regulations capping the maximum residue levels (MRL) of lambda-cyhalothrin, the active ingredient in the most widespread riceworm control utilized by the cultivated wild rice industry. While additional modes of action (e.g. Pyrethrums, Synthetic Pyrethroids and Anthranilic Diamides) available for managing insects in WR, many are problematic. Synthetic Pyrethroids may well come under regulatory restriction in the EU in the near future. Azadirachtin, pyrethrins I & II and pyrethrums do not have sufficient residual activity to be effective management tools. These products break down readily after application, meaning activity is limited only to contact. Using these active ingredients, multiple applications would be required to adequately control these insect pests. The extended period over which adults of both Lepidoptera species enter the field to mate and lay eggs dictates an insecticide must have some residual activity for management to be effective. Lambda Cyhalothrin provided this residual activity making insect management in wild rice effective and sustainable. Evaluation of alternative and unregistered insecticides is, therefore, necessary to provide data for potential IR4 registrations. In this way, this proposal will address the economic and environmental sustainability of WR production.
An additional concern in assessing insecticide efficacy in WR is the application technique utilized in experiments. The aquatic nature of WR production generally dictates aerial application of management inputs. However, most insecticide trials in WR have traditionally been applied using a backpack sprayer and boom; difficult to direct and maintain constant application rates in flooded plots, these sprayers may not truly reflect the application coverage or efficacy of aerial application. Research techniques for this unique crop should be refined to provide the best recommendations to our stakeholders.
Insecticide formulation may also play a significant role in optimizing the efficacy of insecticides in this environment. For example, the Anthranilic Diamides are systemic and may assist in minimizing insecticide applications while still providing adequate management. The difficulty is delivering the insecticide in a formulation that is easily taken up by the plant. Various formulations should be examined for optimizing delivery and minimizing applications.
Due to the aquatic nature of the crop, aerial application is the standard in commercial production or WR. Research applications, however, have typically employed backpack and boom application. The coverage and canopy penetration resulting from this method may well provide a very different assessment of an insecticide’s efficacy than realized by commercial aerial application. A more accurate method of assessing commercial results would be to mirror aerial application in research activities. Small Unmanned Aerial Systems (sUAS), or drones, can be modified to carry and apply small quantities of insecticide and have been used in commercial rice production in Japan’s terraced slopes (difficult to get to with a tractor or commercial ground application equipment) for a number of years. In addition, the remote application of insecticide means research applicators are not exposed at the time of application, nor is there the difficulty and increased chance of accidental spill from attempting to use a backpack applicator in uneven, aquatic habitats. Consequently, using sUAS to apply insecticides to aquatic research plots may increase worker/researcher safety.
Goals and Objectives
- Assess alternate and unregistered insecticides for management of riceworm (Apamea apamiformis [Guenee]) in cultivated wild rice and develop a database to facilitate potential future IR4 registrations
- Assess and compare typical research application technologies to best replicate expected results obtained from commercial production practices
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- MacKay, M. R., and E. W. Rockburn. 1958. Notes on life-history and larval description of Apamea apamiformis (Guenée), a pest of wild rice (Lepidoptera: Noctuidae). Canadian Entomologist 90: 579-582.
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