Tropic ageratum is an annual, distributed over lower mountains, hills, and flatlands in areas south of the Yangtze River where it grows rapidly to a height of 50 to 100 cm. The plant has downy hairs which cover the entire plant, i.e. stem, leaves, and flowers and provide a preferred oviposition site for predatory mite species. Apparently, the down is similar to the silk of the prey mites. The pollen of ageratum and psocids (tiny winged insects), which are found on its leaves, provide food for the predatory mites. These alternate food sources are valuable for sustaining the predatory mite population after they have consumed the mites on the citrus trees.
In the Guangzhou Canton, tropic ageratum is planted in early April using 15 to 30 g of seed/ha. The seed is drilled into the ground between rows of trees, leaving a path down the center for workers. When the plants have seven to eight leaves, the predatory mites leave the soil and other ground covers, and move on to the ageratum. As spring progresses, the predatory mites move on to the citrus trees. To hasten their move to the trees, where pest mite populations are found to be high, ageratum plants can be cut and hung over the citrus branches and left there for at least one day.
In addition to its beneficial role in pest control, ageratum can be grown as a cover crop to provide soil cover, protect against erosion and improve the micro-environment of the orchard by lowering the temperatures and increasing relative humidity. In the winter, ageratum provides shelter for a range of predatory mite species. In the early spring, larger numbers of predators are available in orchards with cover crops compared to those without.
When using Ageratum as a cover crop in citrus orchards, predatory mite populations can be higher than pest populations of P. citri. In some studies predatory mites have occurred at densities of 0.5 to 2.3 per leaf, while pest mites occurred at 0.2 to 1.0 per citrus leaf.
In Jiangxi Province, studies comparing released and ageratum-grown mites showed that both techniques were equally superior to chemical treatments. Scientists have found that A. okinawanus was able to completely control the red mite population of citrus trees in 21 days. Of course, cutting and moving ageratum to the citrus trees reduces the need to artificially rear and release predatory mites.
In the Guangzhou area tropic ageratum has been used for four years within IPM programs. In 1986, 2,000 ha of citrus were treated by hanging ageratum branches to control for pest mites. The ageratum treatment reduced annual pesticide costs by $36 to $60/ha. The net increase in income was 3600 yuan/ha ($393/a) when compared to chemically-controlled orchards. The quality of the fruit was also higher in the ageratum treatment producing 85% first class fruit compared to 65% for the chemically treated orchards.
Tropic ageratum holds many promising applications for use in sustainable agricultural systems in areas where it grows.