Response surface methodology reveals proportionality effects of plant species in conservation plantings on occurrence of generalist predatory arthropods

Authors: Patt, Joseph - Joe; Tarshis Moreno, Aleena; Niedz, Randall

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2020
Publication Date: 4/29/2020
Citation: Patt, J.M., Tarshis Moreno, A.M., Niedz, R.P. 2020. Response surface methodology reveals proportionality effects of plant species in conservation plantings on occurrence of generalist predatory arthropods . PLOS ONE. 15:e0231471. https://doi.org/10.1371/journal.pone.0231471.
DOI: https://doi.org/10.1371/journal.pone.0231471

Interpretive Summary: Our study examined whether a statistical method called response surface methodology (RSM) could be used to optimize mixtures of plants to support the insect predators of the Asian citrus psyllid. The Asian citrus psyllid is a tiny insect that is the vector of citrus greening disease. This disease is fatal to citrus trees and no therapies currently exist to treat it. Citrus greening disease is devastating the citrus industry in Florida and threatens the citrus industry in Texas and California. Our experiment included 14 treatment groups, each comprised of six planters and having a proportion of 0.00, 0.17, 0.33, 0.66, or 1.00 of each plant species. The response we measured was the frequency of psyllid predators trapped on sticky card traps placed in each group and replaced 2 times/week. The following plant species were used: Spring 2017: crown-of-thorns, lima beans, and wild poinsettia; Summer 2017: crown-of-thorns, flowering buckwheat, and partridge pea and, Summer 2018:crown-of-thorns, buckwheat, and ornamental portulaca. Predator occurrence was influenced by: 1) Linear mixture effects, which indicated that predator occurrence was driven by the amount of a single plant species in the mixture; or, 2) Nonlinear blending effects, which indicated that the plant mixture itself had emergent properties that contributed to predator occurrence. Predator abundance was highest in the Spring 2017 experiment and both linear mixture effects and nonlinear blending effects were observed. Predator occurrence decreased in subsequent experiments, which were conducted in the warmer summer months. In both the Summer 2017 and 2018 experiments, only linear mixture effects were observed, indicating that predator occurrence was driven by the amount of a single plant species in the test mixtures. The results showed that not only did the species composition of a plant mixture drive predator occurrence but that proportionality of species could contribute to the outcome as well. This suggests that, when formulating a plant mixture to aid in biological control of the psyllid, then consideration should be given to the proportion of each plant species included in the mixture. RSM can be an important tool for achieving the goal of optimizing mixtures of plants for conserving the natural enemies of the Asian citrus psyllid.

Technical Abstract: Multivariate geometric designs for mixture experiments and response surface methodology (RSM) were tested as a means of optimizing plant mixtures to support predators of the Asian citrus psyllid (Diaphorina citri). The mixture design included 14 treatment groups, each comprised of six planters and having a proportion of 0.00, 0.17, 0.33, 0.66, or 1.00 of each plant species. The response variable was the frequency of predators trapped on sticky card traps placed in each group and replaced 2 times/week. The following plant species were used: Spring 2017: Euphorbia milii, Euphorbia heterophylla, and Phaseolus lunatus; Summer 2017: Euphorbia milii, Fagopyrum esculentum, and Chamaecrista fasciculata; and, Summer 2018: Euphorbia milii, Fagopyrum esculentum, and Portulaca umbraticola. Predator occurrence was influenced by: 1) Linear mixture effects, which indicated that predator occurrence was driven by the amount of a single plant species in the mixture; or, 2) Nonlinear blending effects, which indicated that the plant mixture itself had emergent properties that contributed to predator occurrence. Predator abundance was highest in the Spring 2017 experiment and both linear mixture effects and nonlinear blending effects were observed. Predator occurrence decreased in subsequent experiments, which were conducted in the warmer summer months. In both the Summer 2017 and 2018 experiments, only linear mixture effects were observed, indicating that predator occurrence was driven by the amount of a single plant species in the test mixtures. The results showed that not only did the species composition of a plant mixture drive predator occurrence but that proportionality of species contributed to the outcome as well. This suggests that, when formulating a plant mixture to aid in conservation biological control consideration should be given to the proportion of each plant species included in the mixture. RSM can be an important tool for achieving the goal of optimizing mixtures of plants for conservation biological control.