There are two types of rust mites. The citrus rust mites (CRM) and the pink citrus rust mites (PCRM). The CRM are one of the biggest pests of citrus in our area on fresh fruit. I don’t know that I have seen the PCRM in Georgia but they are probably here. Every year CRM are a problem at the JL Lomax citrus plot. Populations can build up very quickly. After spraying Abamectin plus oil in early May for red spider mites and CRM (I saw very few CRM at this time), I was surprised to see a large population of CRM on the foliage this week. The Abamectin/oil combination controlled the spider mites very well so to see the CRM population this soon after the application was unexpected. Rust mites can develop from an egg to an adult in 6 days at 81 degrees and we have been very warm this May. Each leaf that I examined on the Tango mandarins had at least 10 cream colored mites. I didn’t see many on the Tango fruit but with a high population on the leaves, they will surely move to the fruit. CRM populations build from May until July and decline late August. They can build back up in October and November but populations will not be as high. CRM tend to prefer the lower parts of the tree on the north side which tends to be the more shaded areas. When CRM feed on growing fruit they destroy epidermal cells which fracture creating a rough surface (Fig. 1). When they feed on mature fruit it creates a brown stain but does not destroy epidermal cells. This results in a polished brown look referred to as “bronzing” (Fig. 2)
Fig. 1. Rust mite damage in June.
Fig. 2. Rust mite damage in November.
Rust mites are very small and hard to detect (even when magnified under a hand lens), so unless you know what you are looking for, they are easy to miss. Often several miticide applications are needed to control CRM. Mixing an oil with miticide applications is recommended as oil helps control the eggs. There are many miticides listed in the Florida Citrus Production Guide, https://edis.ifas.ufl.edu/publication/CG002. Good tree coverage is necessary, especially if the tree foliage is dense, so make sure to use at least 150-200 gallons of water per acre and rotate miticides with different modes of action.
