Role of plant species proportionality in conservation biological control

Multivariate geometric designs for mixture experiments and response surface methodology (RSM) were tested as a means of optimizing plant mixtures to support generalist predators.  The mixture design included 14 treatment groups, each 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 traps placed in each group and replaced 2 times/week.  Plant species used were: Spring 2017: Euphorbia milii, E. heterophylla, and Phaseolus lunatus; Summer 2017: E. milii, Fagopyrum esculentum, and Chamaecrista fasciculata; and, Summer 2018: E. milii, F. esculentum, and Portulaca umbraticola.  Predator occurrence was influenced by: 1) ‘Linear mixture effects’, the amount of a single plant species in the mixture; and, 2) plant species proportionality (‘Nonlinear blending effects’), where the plant mixture itself had emergent properties that contributed to predator occurrence. In Spring 2017 both linear mixture effects and nonlinear blending effects were observed.  In the Summer 2017 & 2018 experiments, only linear mixture effects were observed; predator occurrence was driven by Euphorbia milii in 2017 and Portulaca umbraticola in 2018. The results showed that when formulating plant mixtures for supporting generalist predators, the proportion of each plant species included in the mixture may be an important consideration. RSM can be an important tool for optimizing mixtures of plants for conservation biological control.