Classical Biological Control of the Citrus Leafminer in Florida

By: M. A. Hoy and R. Nguyen, University of Florida, IFAS, Entomology and Nematology Department, Gainesville, and Division of Plant Industry, Gainesville

The citrus leaf miner (CLM), Phyllocnistis citrella Stainton, a gracillariid moth, has established and spread rapidly throughout the citrus-growing areas of Florida. Ultimately, a variety of pest management tactics will have to be employed to manage the CLM, including cultural, chemical and biological controls. Relying solely on chemical control is expensive and unlikely to be feasible for very long because this pest can develop resistance to pesticides. Biological control by parasites already present in Florida may provide some control (J. Pena, UF-IFAS, Homestead and P. Stansly, UF-IFAS, Immokalee, personal communications). However, it seems unlikely that the generalist parasites already present in Florida will provide adequate control of the CLM.

Adult citrus leafminer, Phyllocnistis citrella Stainton, in resting pose.Classical biological control, which involves the importation and establishment of host-specific parasites, could provide substantial control of the CLM. Classical biological control efforts in the last three years by Australian scientists have resulted in the establishment of three species of parasites against CLM. While only recently established, these parasites are producing high rates of parasitization (up to 80-90%) in some sites. These parasite species were obtained from China and Thailand and evaluated in Australia for host specificity before being released there.

The three parasites, Citrostichus phyllocnistoides (Narayanan), Cirrospilus quadristriatus Subba Rao & Ramamani, and Ageniaspis citricola Logvinovskaya, are expected to establish in Florida once they are imported and released. Both the Asian collection sites and the Australian establishment sites have a climate that is similar to Florida. Just as important, we have scientific names for these parasites and information about their host specificity. This information is required to obtain permission from the United States Department of Agriculture, Animal and Plant Health Inspection Service (USDA-APHIS) to release these parasites in Florida. Classical biological control programs operate under strict guidelines to prevent the introduction of pests, diseases, and hyperparasites in the USA. Hyperparasites are parasites of parasites and could dramatically reduce the effectiveness of host-specific parasites of the CLM. Host-specific parasites usually provide more effective biological control than generalists.

Much less information is available about the other parasites listed as occurring on the CLM. For example, parasites recently collected from the CLM in the Bahamas by R. Nguyen are known only by their generic name (Zagrammosoma sp., Closterocerus sp., Elasmus sp., Pnigalio sp.) at this time. During November and December 1993, they parasitized up to 45% of the CLM, but by January and February their incidence was reduced to about 5%, and thus their efficacy is not resolved. It is impossible to import and release these species without knowing their full scientific name. It also will be necessary to obtain detailed information about their biology, because these Bahaman parasites probably have a broad host range. Some may be hyperparasites and could reduce the benefits of host-specific parasites of the CLM. Importing the Bahaman species, therefore, is a lower priority than importing the three host-specific species from Australia.

Steps in a Classical Biological Control Project

Classical biological control projects involve a series of steps, usually requiring several years to complete:

  • Identify target pest and its origin, which has been accomplished.
  • Identify potential effective natural enemies of the pest in its center of geographic origin. About 30 species of parasites have been identified as occurring on the CLM, but detailed information on host specificity, biology, and effectiveness is available for only a few. We know the most about the three parasites released in Australia.
  • Obtain parasites, by foreign exploration and by shipment from scientists in China, Thailand, and Australia. Cooperating Australian scientists Dan Smith and Dan Papacek have agreed to help M. A. Hoy obtain material from a region in Australia which matches the climate of Florida very closely, based on weather data in a computer program called CLIMEX. During April 1994, M. A. Hoy will travel to Australia to collect parasites, observe rearing methods, and evaluate the impact of the parasites in citrus groves there. A series of governmental permits must be obtained to import parasites into the USA, including permits from the State of Florida, USDA-APHIS, and the Australian government. For example, the Lacey Act requires us to obtain permits from Australia in order to export these parasites so that rare and endangered Australian species are protected.
  • Evaluate parasites in quarantine. Parasites imported into the USA must be reared through at least one generation in quarantine to prevent release of diseases or hyperparasites harmful to citrus or to parasites of the CLM, respectively. Permission to remove parasites from quarantine must be obtained from both the USDA-APHIS and the Florida Division of Plant Industry. USDA-APHIS guidelines for releasing parasites into the environment are undergoing revision and there is uncertainty as to how much information we will have to provide.
  • Appropriate rearing methods must be developed to maintain the parasites in quarantine. Based on discussions with the Australian scientists, a rearing scheme is being devised and Ru Nguyen will provide assistance in quarantine procedures and permits. A permit to release the parasites into Florida citrus groves cannot be obtained until we have completed studies of these parasites in quarantine and obtained verification that these species are correctly identified. Permission to release parasites into Florida citrus groves may be a bottleneck because the guidelines for releasing foreign species of parasites are under revision and other classical biological control projects have experienced unexpected delays in obtaining permission to release parasites. It is unclear what types of studies may be required to obtain permits to release the parasites into Florida citrus groves.
  • Rear parasites in the laboratory. Rearing will require producing soft, tender citrus foliage for the CLM on small citrus trees throughout the year. Each parasite species will have to be reared separately. Each parasite species has a different biology, which has implications both for rearing and for their impact on CLM in the orchard. For example, Ageniaspis is an internal parasite of CLM larvae; interestingly, no males of Ageniaspis are known. From two to six parasites can develop on a single host. Citrostichus phyllocnistoides is an external parasite of middle and later instar CLM larvae. Females locate CLM larvae and paralyze their host by stinging them with their ovipositor. After the egg is deposited and develops in the CLM larva, the parasite pupates within the leaf mine and the adult parasite emerges from the citrus leaf by cutting a hole through the leaf epidermis. Cirrospilus quadristriatus is an external parasite on late instar CLM larvae and single parasite develops and emerges from the CLM pupal chamber at the edge of the leaf.
  • Rearing of parasites for release into citrus groves will require developing a coordinated method for producing young trees in the appropriate state, the appropriate life stages of CLM (e.g. eggs, small larvae, or larger larvae) appropriate to each parasite species, and pure vigorous cultures of each parasite species. The citrus trees must be free of pests and pesticides in order to maintain pure colonies.
  • Release parasites in citrus groves. Release sites will be selected by Jorge Pena (UF-IFAS, Homestead), Phil Stansly (UF-IFAS, Immokalee), Joe Knapp and Harold Browning (UF-IFAS, Lake Alfred), and Robert Bullock (UF-IFAS, Ft. Pierce). Once releases are made, they will evaluate the effectiveness of the parasites. Because we cannot rear truly large numbers of parasites in our rearing facilities, we plan to increase the number of parasites in pesticide-free citrus nurseries or groves and distribute the parasites to all regions of the state from these initial release sites.
  • The first releases of parasites could take place during the late summer of 1994. That assumes we can collect parasites in Australia, clear them from quarantine in Gainesville within a one or two generations, mass rear them successfully, and obtain permission from the regulatory agencies to release.

Benefits to Florida Citrus Growers

Classical biological control of CLM offers the promise of providing long term and relatively inexpensive control of the CLM. Biological control can reduce production costs by reducing pesticide applications, and is an essential component in a pesticide-resistance management program for CLM. Whether natural enemies can provide sufficient control of the CLM in both nursery trees and in mature groves remains to be determined. Historically, host specific-natural enemies from climatically-similar regions have had a high rate of establishment in the new environment. Once established, they could provide substantial control of the pest within three to five years after their introduction especially if pesticide applications are not so frequent that the parasites cannot survive. It is unlikely that classical biological control will provide the full solution to this new and destructive pest of Florida citrus, particularly in nurseries. Most likely, a combination of pest management tactics will be required in nurseries, including exclusion by screening, flush management, modification of fertilizer applications, and the use of selective pesticides.

This article originally appeared in Citrus Industry magazine, April 1994. Reproduced with permission from the editor.