Recipient Organization
MICHIGAN STATE UNIV
(N/A)
EAST LANSING,MI 48824
Performing Department
ENTOMOLOGY
Non Technical Summary
Herbivore-resistant crop varieties should be the cornerstone of sustainable pest management programs. Some crop species are known to have a diversity of phenotypes that each confer a unique defensive trait against herbivores. Recent studies suggest that deployment of a diversity of defensive phenotypes may be a more effective pest management strategy than deploying a single phenotype that confers a single defensive trait. Moreover, the efficacy of deploying crops with intraspecific defense trait diversity for pest control may be further enhanced by increased mortality from natural enemies. The goal of this project is to uncover the consequences of intra-crop defense diversity in tomato (Solanum lycopersicum) in order to improve management of two economically important agricultural pests: cabbage looper (Trichoplusia ni) and green peach aphid (Myzus persicae). This goal will be addressed by measuring the effects that multiple genetic lines of tomato, each conferring unique resistance via glycoalkaloids, terpenes, or acyl sugars, have on pest performance, movement, population dynamics, and interactions with natural enemies. Experiments will be conducted in the lab to uncover the physiological effects of defense diversity, and in the field to measure pest movement and population dynamics and effects on tomato crop health. Additionally, field studies will identify how defense trait diversity influences interactions between pests and natural enemies by combining treatments with important biocontrol predators. This project will advance the goals of the program area priority by leading to the development of intra-crop defense diversity as an innovative, environmentally-sound, and cost-effective strategy for managing agriculturally important pests.
Animal Health Component
(N/A)
Research Effort Categories
Basic
100%
Applied
(N/A)
Developmental
(N/A)
Goals / Objectives
Our proposal has five objectives. Objectives 1-4 test the physiology, movement, population dynamics, and enemies hypotheses, scaling from laboratory bioassays to large field experiments. In these objectives, we use M82 and the three introgression lines discussed above (IL1-1, IL1-3, and IL10-3) to ask how the effects of each of the focal resistance trait axes--glycoalkaloids, acyl sugars, and terpenes--differ for each of the hypotheses, and how diversity along multiple axes simultaneously changes the consequences of diversity for physiology, movement, population dynamics, and interactions with natural enemies. Objective 5 continues the development of the M82 × pennellii introgression lines as a model system for studying the consequences of intra-crop diversity for insects. In this objective, we phenotype each IL for its internal leaf chemistry, a key component of the plant resistance phenotype that remains to be characterized.Objective 1: Understand the effects of intra-crop defense diversity on pest physiology and responses to predators. We will test the Physiology Hypothesis by rearing cabbage loopers and green peach aphids in the lab on single lines (of M82, IL1-1, IL1-3, and IL10-3) or on mixtures of these lines representing different types and amounts of trait diversity. We will measure insect physiology and performance in each plant diversity treatment. We will test the second part of the Physiology Hypothesis--that plant defense diversity reduces the physiological ability of herbivores to defend against predators--by challenging the pests with predators used in biological control and measuring rates of successful predation. These lab experiments will reveal the potential effects of defense diversity when individual herbivores consume diverse plant defenses.Objective 2: Understand the effects of intra-crop defense diversity on pest movement. We will test the Movement Hypothesis by planting M82 and the three ILs in monocultures and polycultures in open plots in the field and tracking colonization by the entire insect herbivore community, including cabbage loopers and green peach aphids, both of which are present at the field site (MSU Kellogg Biological Station). The polycultures will represent different types and amounts of trait diversity, allowing us to reveal the relationship between herbivore immigration and intra-crop diversity.Objective 3: Understand the effects of intra-crop defense diversity on pest population dynamics and predator-prey interactions. We will test the Population Dynamics Hypothesis by tracking population dynamics of cabbage loopers and green peach aphids for multiple generations in large field cages with monocultures and polycultures of M82 and the three ILs, as above. Measurements of pest survival, fecundity, and density will allow us to model population dynamics in the presence and absence of plant defense diversity, indicating the total effect of defense diversity on pest control.Objective 4: Understand the effects of intra-crop defense diversity on recruitment of natural enemies. We will test the Enemies Hypothesis by tethering cabbage loopers or bagging aphids in field plots of monocultures and polycultures of M82 and the three ILs and tracking predation events and colonization by natural enemies.Objective 5: Characterize the internal insect-resistance traits of M82 and all 76 ILs. The chemistry of resistance traits in the IL trichomes and epidermal cells has been extensively characterized both in terms of volatile and non-volatile components (Schilmiller et al. 2010). The final objective of this proposal is to continue the development of the M82 × pennellii introgression lines as a model system for studying the consequences of intra-crop diversity for insects. We will do this by phenotyping each IL for its internal leaf chemistry, a key component of the plant resistance phenotype that remains to be characterized. This will include glycoalkaloids, phenolics, protease inhibitors, nutrient content, leaf water content, and leaf toughness. Finally, we will assay each IL individually with cabbage loopers and green peach aphids to link plant phenotypes to pest performance.
Project Methods
Introgression LinesObjectives 1-4 rely on M82, IL1-1, IL1-3, and IL10-3 of the tomato cv. M82 × Solanum pennellii introgression line system, as discussed above. We chose IL1-1 from among the 76 ILs because it displays a differing glycoalkaloid resistance phenotype from M82, and is similar to M82 in other resistance traits. Glycoalkaloids are a key class of secondary metabolites for resistance against insect pests in tomato and other solanaceous plants (Tingey 1984, Friedman 2002). In particular, IL1-1 has several unique glycoalkaloids, including two isomers of dehydrotomatine, didehydrotomatine, and hydroxytomatine (Schilmiller et al. 2010). IL1-1 has lower abundances of tomatine and other glycoalkoids common in M82. We chose IL1-3 because it has a different profile of acyl sugars, many of which lack acetyl groups relative to M82. Acyl sugars are key resistance compounds found in Solanum trichomes and trichome exudates, and which influence a wide range of insect pests from sucking to chewing insects (Leckie et al. 2012, Lucatti et al. 2013). Finally, we chose IL10-3 because it has a different terpene profile from M82, with especially low abundances of several major sesquiterpenes, including δ-elemene, caryophyllene, and α-humulene. Terpenes in general, and sesquiterpenes specifically, are major constituents of trichome chemistry, and in Solanum they play key roles in insect resistance, host-location behavior, and attraction of natural enemies (Bleeker et al. 2012). Together these lines and M82 represent diversity along three major resistance trait axes. Remaining resistance trait axes will be quantified and bioassayed as part of Objective 5.General ApproachThe experiments for Objectives 1-4 follow the same general format: four monocultures and seven polycultures presented to insect pests in the presence or absence of natural enemies in lab or field settings. The monoculture treatments are M82, IL1-1, IL1-3, or IL10-3 in isolation, and the polyculture treatments are all pairwise combinations of M82 and ILs plus all three ILs (Table 1). Examining the insect response variables in the monocultures allows us to see the consequences of all the lines (and their respective phenotypes) in the absence of diversity. Each polyculture treatment represents one or more different axes of trait diversity (Table 1). For example, growing M82 in polyculture with IL1-1 gives intra-crop diversity in glycoalkaloids (Fig. 6). Growing IL1-1 and IL1-3 gives diversity in both glycoalkaloids and acyl sugars. Growing all three ILs together gives diversity along all three trait axes. It is also important to note that the polycultures, in addition to giving a controlled amount of trait diversity, also give a controlled amount of genetic diversity, which we can map directly to the tomato genome. For example, the polyculture with M82 and IL1-1 has intra-crop genetic diversity only in region 1 of chromosome 1. Moreover, the pennellii chromosomal substitutions are similar in size across ILs. This means that all pairwise polycultures have the same amount of genetic diversity; they differ only in the genomic location of the diversity. This allows us to pinpoint where in the genome intraspecific diversity is most influential for the ecology of higher trophic levels, a major advance for the study of the ecological consequences of plant diversity.Table 1. Monoculture and polyculture treatments and their trait diversity levels.Plant treatmentTrait diversityM82None (M82 for all traits)IL1-1None (unique glycoalkaloid profile)IL1-3None (unique acyl sugar profile)IL10-3None (unique terpene profile)M82 + IL1-1Glycoalkaloid diversityM82 + IL1-3Acyl sugar diversityM82 + IL10-3Terpene diversityIL1-1 + IL1-3Glycoalkaloid and acyl sugar diversityIL1-1 + IL10-3Glycoalkaloid and terpene diversityIL1-3 + IL10-3Acyl sugar and terpene diversityIL1-1 + IL1-3 + IL 10-3Diversity along all three trait axesWe will interpret the outcome of all polyculture-monoculture experiments by comparing response variables (e.g., insect performance or population dynamics, plant damage, natural enemy recruitment) in a polyculture to the mean of those response variables across the monocultures represented in the polyculture. If the trait diversity in a given polyculture negatively influences insect performance, then insect performance in that polyculture will be significantly lower than the mean performance across the monocultures represented in that polyculture. We will make these comparisons for each individual diversity axis as well as for polycultures with multiple diversity axes.Our proposed work is focused on how intra-crop trait diversity directly influences insect pests and natural enemies, but intra-crop diversity could also indirectly influence insects by changing plant-plant competition (Moreira et al. 2015). This mechanism does not apply to Objectives 1 and 5 because the plants will be grown in pots and will thus not be competing for resources . Our methods for field experiments reduce the likelihood of plant competition by adequately spacing plants and fertilizing and irrigating as needed, thus making this mechanism less important. Regardless, we still test for the existence of competition effects by repeating diversity treatments in Objectives 2 and 4 using greenhouse-grown plants sunk into the ground in pots, preventing resource competition. If these experiments identify effects of intra-crop diversity on plant-plant competition, we will repeat Objective 3 using potted plants, which will allow us to tease apart the direct effects of diversity on insects from the indirect effects that occur via diversity altering plant-plant competition.