National and international quarantines are critical to minimize the potential of long-distance dissemination. Integrated pest management strategies are needed such as:
- early detection,
- chemical and biological control of the vectors,
- potential chemical control of the Liberibacters using chemical and antibiotic treatments of trees,
- isolation and protection of budwood sources and plant propagation in screened-in, insect-proof locations, and
- development of resistant cultivars.
Huanglongbing has a complex pathosystem (an ecosystem based on parasitism). There are multiple strains, diverse hosts, several insect vectors, and different environmental conditions that affect the expression and spread of the disease. Three forms of the disease are known (Asian, African, and American), and these are associated with different species and strains of Liberibactors that are disseminated by different species of citrus psyllid insect vectors.
Vectors of the Liberibacters include citrus psyllids Diaphorina citri and Trioza erytreae. Huanglongbing can also be spread by grafting. Transmission can occur in the nursery and in the orchard.
Non-citrus hosts have been identified as alternate hosts for the Liberibacters and the citrus psyllids, but their role is still unclear in the epidemiology of the disease.
Huanglongbing (HLB) is a major disease of citrus that has caused catastrophic damage to citrus trees worldwide. The disease causes reduced fruit quality and yield, tree decline, and eventual tree death.
Symptoms are variable and can resemble several disorders of citrus. Typical symptoms include:
- yellow shoots with pale green and yellow flushes;
- non-symmetrical mottled leaves (shades of yellow and green on either side of the mid-rib);
- thickened, leathery leaves;
- enlarged, corky mid-ribs of leaves; and
- leaves with zinc deficiency symptoms that include upright leaves in relation to the shoot (acute shoot-leaf angles).
Defoliation, fruit drop, and shoot dieback occurs in more advanced stages. Young trees may die soon after infection; whereas older trees may die in seven to nine years after infection.
Fruit symptoms include small, misshaped fruit that are lopsided or asymmetrical and exhibit color inversion from yellow to orange to green on the peduncle side while remaining green on the stylar end. The vascular tissue is brownish at the peduncle side of fruit. Seeds of affected fruit are small, brown, and aborted.
Predators kill and feed on several to many individual prey during their lifetimes. Many species of amphibians, birds, mammals, and reptiles prey extensively on insects.
Predatory beetles, flies, lacewings, true bugs (Order Hemiptera), and wasps feed on various pest insects or mites. Most spiders feed entirely on insects.
Predatory mites that feed primarily on pest spider mites include Amblyseius spp., Neoseiulus spp., and the western predatory mite, Galendromus occidentalis.
Natural enemy pathogens are microorganisms including certain bacteria, fungi, nematodes, protozoa, and viruses that can infect and kill the host.
Populations of some aphids, caterpillars, mites, and other invertebrates are sometimes drastically reduced by naturally occurring pathogens, usually under conditions such as prolonged high humidity or dense pest populations.
In addition to a naturally occurring disease outbreak (epizootic), some beneficial pathogens are commercially available as biological or microbial pesticides.
These include Bacillus thuringiensis or Bt, entomopathogenic nematodes, and granulosis viruses.
A parasite is an organism that lives and feeds in or on a host. Insect parasites can develop on the inside or outside of the host’s body. Often only the immature stage of the parasite feeds on the host. However, adult females of certain parasites (such as many wasps that attack scales and whiteflies) feed on and kill their hosts, providing an easily overlooked but important source of biological control in addition to the host mortality caused by parasitism.
Although the term “parasite” is used here, true parasites (e.g., fleas and ticks) do not typically kill their hosts. Species useful in biological control, and discussed here, kill their hosts; they are more precisely called “parasitoids.”
Most parasitic insects are either flies (Order Diptera) or wasps (Order Hymenoptera). Parasitic wasps occur in over three dozen Hymenoptera families. For example, Aphidiinae (a subfamily of Braconidae) attack aphids. Trichogrammatidae parasitize insect eggs. Aphelinidae, Encyrtidae, Eulophidae, and Ichneumonidae are other groups that parasitize insect pests. It’s important to note that these tiny to medium-sized wasps are incapable of stinging people. The most common parasitic flies are the typically hairy Tachinidae. Adult tachinids often resemble house flies. Their larvae are maggots that feed inside the host.
Biological control is the beneficial action of parasites, pathogens, and predators in managing pests and their damage.
Biocontrol provided by these living organisms, collectively called “natural enemies,” is especially important for reducing the numbers of pest insects and mites.
Use of natural enemies for biological control of rangeland and wildland weeds (e.g., Klamath weed, St. Johnswort) is also effective.
Plant pathogens, nematodes, and vertebrates also have many natural enemies, but this biological control is often harder to recognize, less well understood, and/or more difficult to manage. Conservation, augmentation, and classical biological control are tactics for harnessing natural enemies’ benefits.
Spinosad is a natural substance made by a soil bacterium that can be toxic to insects.
It is used to control a wide variety of pests. These include thrips, leafminers, spider mites, mosquitoes, ants, fruit flies and others.
Many products containing spinosad are used on crops and ornamental plants.