Leaves of the Curry tree (Murraya koenigii) are a key ingredient of many Indian dishes. While handling curry leaves in my kitchen, I noticed some brownish spots under the leaves (Fig. 1). This triggered my curiosity and I observed these spots under the foldscope.
The brown spot turned out to be an insect with wings, legs, prominent antennae and reddish eyes (Fig. 2). With this description I searched on the internet to identify it. At first I thought it may be an aphid. Then looking at these images of the life cycle of the Asian citrus psyllid (Diaphornia citri kuwayama) I concluded that it could be the late nymph stage of the psyllid.
Over the next few days I continued my explorations with some more leaves. I saw one white sticky web under a curry leaf, and inside the web I noticed some white-coloured patches (Fig. 3). I observed these white patches through my foldscope and this is what I saw (Figs. 4, 5).
A few day later in another batch of curry leaves I observed black spots on both the sides of the leaves, which looked like this under the foldscope (Fig. 6).
I also noticed a woolly patch near the axil of the leaf (Fig. 7), which I observed closely with my foldscope (Fig. 8).
Now my question was, whether the spots and patches which I saw earlier are the same as these three observations (spots on the upper side and woolly patch near the leaf axil) or not! Based on my foldscopic investigations, I concluded that they are different. Black spots found on the upper side of the curry leaf looked to me like some insect’s pupal stage — but I also noticed that the structure had prominent white eyes, many bristle-like appendages, white lines along the length of dorsal side, and some gaps near the thorax region (better seen under a compound microscope). Here are the foldscope pictures from my observations (Fig 9, 10 and 11).
Again I started searching for information on the internet. I checked out images of the various pests of curry leaves but none really fitted the observations. I welcome your suggestions to help me identify this insect. My overall interpretation is that the white woolly patches are probably the eggs-stage of the insects (nymphs), the light brown spots are advanced nymphs, and black spots are the empty pupae shells of some insects yet to be identified. Well, then the question comes, where are the insects? Unfortunately, right now I have only the leaves bought from the market. If I get a chance to see the plant, I will continue my observations to find and identify the insects.
Another few days later, for Pooja (worship) purpose, I got some Mango (Mangifera Indica ) leaves. Out of curiosity I turned the leaf to look at the underside. Oh! What a coincidence, I observed the same whitish web (Fig. 12) and black spots in the Mango leaves (Fig. 13).
I know that curry tree belongs to the citrus family Rutaceae, whereas Mango belongs to the family Anacardiaceae. Is there any possibility that the same kind of pests infect plants belonging to two different families? Yes , it could be possible as there are pests categorized as oligophages, which infect a few (more than 2) plant groups and also, polyphages which can infect many species of plants. Interestingly, I found similar results for the black spots and white sticky web in the Mango leaves, as I found on the curry leaves, when observed under the foldscope.
Also, both the plant leaves showed necrosis, which could be because of viral infection (Fig. 14). It is quite possible that these insects act as vectors for the virus.
Pests of the curry leaves cause significant damage to the yield of the plant. However, by spraying water or soap water with some force and with the help of natural predators, these pests can be controlled.
After three decades as a school teacher these scientific voyages with the foldscope have encouraged me to investigate further and I hope to come up with some interesting conclusions.