Why is it a problem? Feeding on sap of young mandarins can cause considerable early (“summer”) fruit drop, although compensatory development of remaining fruit can lessen the economic importance. Damage can be severe, but management is generally not needed in most years and orchards.
Where and when is it a problem? It is a problem below 1000 m asl, causing fruit drop from April to September. It can cause near complete loss in some areas, possibly up to twice a decade.
Adults are large (2-2.5 cm long) and have bodies that are more or less pentagonal in shape. They have prominent spines on each shoulder of the pronotum. The abdomen carries laterally six pairs of short spines. The colour is rather uniformly green (sometimes more brownish), with lateral rows of dark spots on the abdomen. Eggs are about 1 mm in diameter and initially white, but becomes marked with black and red as they develop. There are five nymphal stages. Early stage nymphs are marked with black, green, yellow, white and orange, and can have a rather black-and-white spotted appearance. Later stages look more like the adults and are mainly green with black markings.
The citrus shield bug causes “Summer Fruit Drop” in mandarins, when the fruits are of about marble size and slightly bigger (from April to September). Dropped fruits can have brown secondary spots around feeding punctures, caused by entry of secondary infection by microorganisms. Usually the fruits have a brownish discoloration in the fruit centre. The seed coat can also be discoloured. Bug attack can also cause shrinkage of the seed cotyledon.
Young nymphs are gregarious. Early in the morning, groups can be seen sitting on the trunks of citrus trees in a neat circle. Adults and nymphs are often observed crowded on tree trunks.
Confusion with other pests: Adults and nymphs are characteristic.
Lifecycle: Adults are in resting stage in winter. In early summer the females lay their eggs in batches of about 15 eggs, usually on the upper surface of citrus leaves, between May to September. The nymphs take 4-6 weeks to develop into adults. During the early stages, nymphs will crowd together on the bark. At altitudes from 500 to 1000 m there is probably only one generation per year. Two generations are possible at lower altitudes. Young nymphs are gregarious.
Dispersal: Adults fly.
When can damage be expected? The citrus shield bug can cause very summer fruit drop in some years in some orchards. However, the causes of such outbreaks is not yet understood.
Hosts: Hosts include citrus and native Rutaceae.
This is an important citrus pest, but losses can normally be minimised by frequent inspection and physical control.
Inspect trees weekly from April to June for young bugs in orchards where citrus shield bug is known to be a problem. Search the trunks of trees in the early morning. Pest numbers can vary considerably between years, so vigilance is important. The action level is 10% or more trees infested with one or more bugs.
Effect of variety
- In April to June young bugs (nymphs) gather early in the morning on the trunks of citrus trees and can be easily killed by hand or a flat piece of wood.
- The most important predator in Bhutan is the red tree ant. They can be encouraged, even increased, by placing ant nests from forest trees in mandarin trees. However, farmers are not keen as they can cause other problems. Nonetheless, ants are an important friend of citrus farmers and should be protected and enhanced.
- Pesticides are not recommended as they interfere with important natural control mechanisms (including egg parasitoids and red tree ants).
- Spot or selective spraying with any contact insecticide can be done when the young bugs group together particularly early in the morning.
Image acknowledgements: NPPC.