A leopard doesn’t change its spots, but these mites will!

If you search “Lewis mite” or “Eotetranychus lewisi” in google scholar you will find less than 450 scientific papers, but if you type in “two-spotted spider mite” or “Tetranychus urticae” you will find over 119,000 papers. Lewis mites appear to be an under researched pest. This year I received my first exposure to this species when a biocontrol trial started early because of a Lewis mite infestation.

Lewis mite is a serious pest because it can outpace two-spotted spider mite (TSSM) and predatory mites when temperatures are between 5-15 °C (Kaur and Zalom 2017). Lewis mite essentially outcompetes TSSM in the winter months. The predatory mite Neoseiulus californicus feeds more successfully on Lewis mite when compared to the commonly used predatory mite Phytoseiulus persimilis (Howell and Daugovish 2013). The problem that many PCAs know is that N. californicus has difficulty establishing and hunting Lewis mite when temperatures are below 15 °C.

**Figure 1.** A *Neoseiulus californicus* adult (right) feeding on a juvenile *Eotetranychus lewisi* nymph (left). Notice the large *N. californicus* egg in the top left corner.

As we were counting mites as part of a research project, my undergraduates would occasionally get confused differentiating between TSSM and Lewis mite. Their confusion reminded me of what I heard from a PCA who said it seemed as if Lewis mites would transform into TSSM. Now I can see why. I shot the two videos below for you to witness the movement of the organelles and food within the body of the mite that create the illusion of changing spots.

Figure 2. A video of Lewis mite (Eotetranychus lewisi) adult mite feeding. Notice the movement of the spots on the body of the mite.

Figure 3. A video of twospotted spider mite (Tetranychus urticae) adult mite feeding. Notice the movement of the middle stripe on the mite body.

We also observed variation in the form and number of spots for TSSM. We found a few mites that were completely dark green or had three spots. Lewis mite appeared to vary their spot size and number as well, with 6 to 8 spots covering the lateral edges of the mite body. There are likely environmental and genetic reasons for this variation, but I have not yet found a source that explains the variation in spots.

If the spots are causing confusion, the size and shape is the best way to confirm mite identity. TSSM has spots near the head and it will be much larger than Lewis mite. TSSM can also be red to orange during the winter and spring. Some will say that these red mites are carmine mite, which is likely not the case anymore. Tetranychus cinnabarinus or carmine mite is actually TSSM (Auger et al. 2013). Genetic and morphological evidence along with the fact that they easily hybridize points to synonymy, when two species are considered to be the same.

**Figure 4**. A two-spotted spider mite nymph showing the orange red coloration typical of an overwintering morph. The color change is due to the proportion of ketocarotenoids in its body.

Lewis has an elliptical shape with spots running the lateral length of the mite body. We have observed 6 to 8 spots on the Lewis mite. We also observed that there are primarily two spots on the backside of the Lewis mite. The spot sizes are also much smaller. The nymphs are also narrower and will occasionally be a neon green based on our observations.

**Figure 5**. A Lewis mite nymph showing that neon green color noted by the author. A Lewis mite egg waits to hatch next to it.

Literature Cited

Kaur P, Zalom FG, 2017. Effect of temperature on the development of Tetranychus urticae and Eotetranychus lewisi on strawberry. J. Entomol. Zool. Studies. 5(4):441-444

Howell AD, Daugovish O, 2013. Biological control of Eotetranychus lewisi and Tetranychus urticae (Acari:Tetranychidae) on strawberry by four phytoseids (Acari:Phytoseiidae) J. Econ Entomol. 106(1):80-85

Philippe A, Migeon A, Ueckermann EA, et al 2013. Evidence for synonymy between Tetranychus urticae and Tetranychus cinnabarinus (Acari, Prostigmata, Tetranychidae): Review and new data. Acarologia 53(4):383-415.