This colored clay protects crops by effectively changing the reflectance of the foliage, camouflaging it to prevent insect attacks. Huanglongbing disease, or citrus greening, is the biggest threat to the $9.3 billion Florida citrus industry. The disease is present throughout Florida’s citrus growing regions but is also found in Texas, California, and Arizona. Diaphorina citri (D. citr) is the insect responsible for the transmission of Huanglongbing disease. These insects are strongly responsive to visual host plant cues. Recent laboratory studies have discovered that D. citri are most attracted to reflected yellow and green light. Available technologies to manage pests and diseases in agricultural crops mainly consist of chemical pesticides, which may be harmful to the environment and non-target species. Researchers at the University of Florida have developed colored kaolin clays that alter the spectral reflectance of coated citrus leaves, preventing D. citri from identifying citrus leaves as a food source. In general, colored clays offer an environmentally friendly option for pest control. Colored clay also can be used in the paper, paint, and cosmetic industries.
Crop protection through optical deterrence of insects
The purpose of the colored clays is to interfere with the visual cues of citrus plants, preventing D. citri from identifying and attacking them. Colored clay coatings change the reflectance of the foliage depending on the spectral profile of the dye used. Dyes having absorbance/reflectance in UV – visible – near infrared range are particularly useful for this application. Combining kaolin clay particles with surfactants and dyes produces the colored clays. The kaolin clay particles are negatively charged on their surface and positively charged on the edges. Positively charged surfactants and polymers added to the kaolin clay particles change the surface charge. After removing excess surfactant, researchers add negatively charged dyes, which are absorbed into the clay. As a result, the dye is non-covalently attached to the surface of the clay particles. The leaching of dye from the clay is minimized by adjusting the dye to surfactant ratio. Researchers investigated the effect of color by examining the number of D. citri found on plants coated with different color kaolin clays, as compared to an untreated citrus plant. Additionally, polymer adjuvants coated on top of the colored clay increase the longevity of the clay when exposed to rainfall.
