The Asian citrus psyllid (Diaphorina citri) is a major insect pest of citrus cultivars, though it may also feed on a wide range of alternative host plants. Recent research has identified the plant Cordia myxa as another alternate host, and researchers are evaluating whether C. myxa might have potential as a trap crop for use in Asian citrus psyllid management. Here, activity of D. citri is shown on upper (left) and lower (right) surfaces of C. myxa. (Photo by Muhammad Arshad, Ph.D.)

By Muhammad Arshad, Ph.D.

The Asian citrus psyllid (Diaphorina citri) is one of the important insect pests of all citrus cultivars and causes severe losses (up to 95 percent) to citrus plants in many countries over the globe. Both nymph and adult stages of the Asian citrus psyllid (ACP) suck the cell sap from flowers, buds, leaves and young shoots. It causes leaf distortion, curling of leaves, complete defoliation, or shedding of flower and leaves. ACP also carries the bacterium, Candidatus Liberibacter asiaticus, that causes citrus greening disease, also known as Huanglongbing (HLB), with yellowing and chlorosis of leaves.

Muhammad Arshad, Ph.D.

The host range of ACP includes more than 25 species in the Rutaceae family (order: Sapindales), and it is well-established on almost all citrus cultivars throughout citrus producing countries. However, ACP may feed on a wide range of alternative host plants other than Rutaceae. While some of these alternative host plants may serve only as a temporary or partial host and may not be suitable for reproduction and growth of insects, these plants are important because they allow the insects to survive in the absence of their preferred hosts.

In March 2019, a new plant was added to this list of alternate hosts, when my colleagues Muhammad Irfan Ullah, Naciye Sena Çağatay, Fatma Dikmen, Asad Abdullah, and Muhammad Afzal and I reported ACP feeding on Cordia myxa plants in the south region of the state of Punjab, Pakistan, published in Southwestern Entomologist.

It was a surprise to me seeing the ACP population on C. myxa plant, as I had never heard of it being found on this host plant globally. However, throughout our observations, I have visited the C. myxa plants regularly and found that ACP is well-established (see image below) and can reproduce on this plant even in warmer months (May-July) in the Southern Punjab state of Pakistan.

Because of the important role of ACP plays as a vector of a plant disease, it is important to identify the species of plants that can act as a host, as these plants can act as bacterial reservoirs. Knowledge about the nature of ACP and the plants they live in can help in the management and control of the bacteria they carry. With each plant that the ACP accepts for feeding and probing, there is an increased risk of ACP acquiring and transmitting the bacteria, and the threat of infection may increase if their immature stages develop on those plants and then spread bacteria to other plant species as adults. Or, the threat of infection may be raised by an increase in the number of ACP at a location.

Meanwhile, over-reliance on chemical controls can lead to insecticide resistance in the long-term, which presents the need for the development of alternative control strategies to suppress populations of ACP, as well as better tools to detect and monitor this bacterial vector. The development of alternative control strategies, such as trap cropping, will require a thorough understanding of ACP ecology and behavior in relation to the findings of host plants.

The Asian citrus psyllid (Diaphorina citri) is a major insect pest of citrus cultivars, though it may also feed on a wide range of alternative host plants. Recent research has identified the plant Cordia myxa as another alternate host, and researchers are evaluating whether C. myxa might have potential as a trap crop for use in Asian citrus psyllid management. (Photo by Muhammad Arshad, Ph.D.)

The Asian citrus psyllid (Diaphorina citri) is a major insect pest of citrus cultivars, though it may also feed on a wide range of alternative host plants. Recent research has identified the plant Cordia myxa as another alternate host, and researchers are evaluating whether C. myxa might have potential as a trap crop for use in Asian citrus psyllid management. (Photo by Muhammad Arshad, Ph.D.)

The Asian citrus psyllid (Diaphorina citri) is a major insect pest of citrus cultivars, though it may also feed on a wide range of alternative host plants. Recent research has identified the plant Cordia myxa as another alternate host, and researchers are evaluating whether C. myxa might have potential as a trap crop for use in Asian citrus psyllid management. (Photo by Muhammad Arshad, Ph.D.)

Use of trap cropping has been studied for the integrated pest management of various insect pests that are important vectors of plant diseases, such as leafhoppers, aphids, planthoppers, and whitefies. In trap cropping, plants are used to attract insects or other organisms away from nearby important economic crops.

To begin evaluating C. myxa‘s role as a host plant for ACP, my colleagues and I studied whether ACP found on C. myxa carried Candidatus Liberibactor asiaticus (CLas), the results of which we published in March 2020 in the Journal of Economic Entomology.

We found that, although ACP can be infected with CLas on C. myxa, bacterial levels were lower in ACP on C. myxa than on citrus cultivars, probably due to the lower ACP reproduction rates in C. myxa.

Although the bacteria were detected in ACP on C. myxa, there is need to study the CLas acquisition and transmission efficiency of ACP in C. myxa plant. If the transmission efficiency is lower in C. myxa plant, that means it would be a good host for ACP but a poor host for CLas,so it may be used as a potential trap crop to attract ACP.

This work opens the possibility of performing further studies determining the potential of C. myxa as a trap crop to manage the ACP population and, ultimately, to reduce HLB disease in citrus crops.

Muhammad Arshad, Ph.D., is an assistant professor (visiting faculty) in the Department of Entomology at the University of Sargodha, Pakistan. Email: makuaf@gmail.com.