Recipient Organization
CORNELL UNIVERSITY
(N/A)
ITHACA,NY 14853
Performing Department
Ecology & Evolutionary Biology
Non Technical Summary
Cucumber is a major fresh market vegetable crop grown in New York State that suffers losses from insect pests and disease. We propose to collaborate with cucurbit breeders to investigate the role of natural defenses (cucurbitacins) in resistance to the striped cucumber beetle, and their indirect effects on disease and pollination as a means of improving natural pest control.
Animal Health Component
80%
Research Effort Categories
Basic
20%
Applied
80%
Developmental
0%
Goals / Objectives
Cucumber is commonly grown in NY (1,800 acres, worth >$10 million), suffers losses from insect pests and disease, and relies on pollinators for fruit set. Cornell has been a leader in cucumber breeding, with the "Marketmore" varieties being widely planted for >30 years, and new varieties continuing to be released (Cavatorta et al. 2007). Our lab has made contributions showing the role of bitter compounds (tri-terpenes called cucurbitacins) in cucumbers impacting direct and indirect resistance to pests (Agrawal et al. 2002, McGuire and Agrawal 2005). The proposed work builds on the need for effective management of pests and pollination in cucumber, and the continued interest of plant breeders in cucurbitacins.In addition to the occasional nuisance bitterness in cucumbers, early research showed that the primary effect of cucurbitacins was to provide resistance to several insect pests, but also susceptibility to the striped cucumber beetle (Da Costa and Jones 1971, Cavatorta et al. 2007). This trade-off has led to the widespread planting of non-bitter varieties (e.g., Marketmore 80, 97), which are less susceptible to striped cucumber beetles. Nonetheless, the mechanisms of resistance / susceptibility are not known, and there is a current need and opportunity to study the direct and indirect impacts of cucurbitacins on beetle attraction, pheromone production (which causes them to aggregate) and ultimately on pollination / yield as well as disease incidence. Benefits of cucurbitacins and their associated effects could be exploited for better integrated pest management. Over the next 3 years, we will study the impacts of varietal variation and breeding of cucurbitacins in cucumber plants resistance of the striped cucumber beetles, and indirect effects on pollination and disease. Specific objectives include:1. Testing the effects of cucurbitacins on attraction versus retention of striped cucumber beetles to cucumber plants.2. Separating the role of cucurbitacins, plant volatiles (induced by damage), and beetle pheromones (produced upon feeding) in the build-up of striped cucumber beetles on plants.3. Addressing the effects of cucurbitacins on yield directly and indirectly through effects on beetles, pollination, and disease incidence. Our ultimate goal is to work towards providing cucurbit growers, primarily small scale farmers, especially those using organic practices, with data on the impacts of beetle attack, pollination and disease for the quality and yield of cucumbers.The role of cucurbitacins in resistance to generalist pests, such as spider mites, was proposed to be dependent on a single compound, cucurbitain C (Da Costa and Jones 1971), and this was later confirmed to be 1) the primary cucurbitacin in cucumber, and 2) quantitatively related to resistance (Balkema-Boomstra et al. 2003). The simplicity of this system (i.e., the lack of diverse compounds implicated in resistance) makes it attractive for study and manipulation. Surprisingly, the quantitative role of cucurbitacins C in susceptibility to Acalymma vittatum, the eastern striped cucumber beetle, appears to be less clear than that previously described for the western spotter cucumber beetle (Diabrotica undecimpunctata) (Metcalf et al. 1982). In particular, work conducted at Cornell over a decade ago revealed a complex relationship between Acalymma and cucurbitacins, but not one that was obviously connected to field susceptibility (Smyth et al. 2002, Smyth and Hoffmann 2003). Accordingly, two gaps occur in the literature: 1) if cucurbitacins are not attractive to the specialist Acalymma, are the net effects positive in terms of resistance to the community of pests (including disease); and 2) are there non-target effects on pollinators, which are avoidable due to the pollination-limitation at least sometimes observed for cucumber. Given recent advances in breeding of cucumbers, and the potential to separate fruit and leaf cucurbitacins (M. Mazourek, personal communication), the time is ripe to revisit this system and work towards an understanding of resistance that could be employed in subsequent breeding.
Project Methods
We will use a three-pronged approach to study the direct and indirect effects of cucurbitacins on beetles, disease, pollinator visitation, and yield: controlled greenhouse trials, field attraction experiments, and open field experiments. All experiments will be conducted with two pairs of cucumber lines: the Marketmore 76/80 near isogenic bitter/sweet pair that our lab and others have previously used (Agrawal et al. 1999, Agrawal et al. 2002, Smyth and Hoffmann 2003, Barrett and Agrawal 2004), and new lines developed by Mazourek's lab at Cornell that differentially express bitterness in leaves versus fruit. Of particular importance will be to characterize the expected results from the classic Marketmore varieties to newly available germplasm.In the greenhouse experiments, plants from each isoline will be treated with beetle damage or left as controls, and will be assayed for 1) beetle attraction using a choice test in a cage, 2) caterpillar performance (Trichoplusia ni, an occasional pest of cucumber), 3) downy mildew establishment, and 4) powdery mildew establishment. In all, each trial will have 4 plant lines X 2 treatments X 4 assay pests X 10 replicates = 320 plants per trial; at least three trials will be run. Cucurbitacins will be assayed in a subset of the plants to test for induced effects. Wild caught spring-emerging adult beetles will be used in the assays because they are most relevant to colonization, and have been shown to exhibit different responses than laboratory reared beetles (Smyth et al. 2002).In the field attraction experiment, cucumber plants from the four lines will be grown in the greenhouse and damaged by beetles or not and then placed in mesh sleeves surrounded by metal cages with sticky trangletrap strips. These cages have been successfully employed several times by Cornell researchers and 25 are available for these experiments (Smyth and Hoffmann 2003). Each trial will thus employ 4 plant lines X 2 treatments X 3 replicates. Trials will be run for one week each, with beetles counted on the sticky strips and volatiles collected from treatments to identify plant volatiles and beetle pheromones. Trials will be repeated 5 times sequentially and will be conducted at the Freeville Experimental Farm where there are established populations of striped cucumber beetles.Finally, open field plots will be established with cucumber plants grown (in Freeville) again with the 4 plant lines and 2 beetle treatments. These plants will be monitored over the growing season for beetles. During peak flowering, pollinator visitation will be recorded on a per plant basis (accounting for plant size and number of flowers). Fruits will be harvested as they mature (and will be enumerated and weighed). At the end of the season, yield and disease incidence will be recorded. Cucurbitacins will be assayed in a subset of the plants. The link between insect behavior in controlled settings, to field to cages, to the net effect in open plots will set the stage evaluating the usefulness of deploying resistance traits.