Occurrence Patterns and Control Methods of the Citrus Whitefly

The citrus whitefly, Dialeurodes citri (Ashmead), also known as the citrus whitefly, Tongcao whitefly, and orange-yellow whitefly. In recent years, damage in citrus orchards across the counties (districts) of northwestern Fujian has been extremely severe, spreading from localized outbreaks to contiguous occurrence, with a worsening trend. Adults and nymphs suck plant sap, causing infested leaves to lose chlorophyll and turn yellow, and secrete large amounts of honeydew that contaminates leaves and fruit, often triggering outbreaks of sooty mold, especially when the fruit is near maturity, coating the fruit surface with a layer of sooty mold and seriously affecting the fruit's appearance and quality, causing relatively large economic losses to fruit growers. It has risen to become a major pest in citrus production.

1. Occurrence patterns

The citrus whitefly produces 5 generations per year in northwestern Fujian, mostly overwintering as 3rd-instar nymphs and pupae, emerging as adults in late February and early March of the following year, mating and laying eggs after 1-3 days, with an average of 142.5 eggs per female. It can also reproduce by parthenogenesis, with offspring being male. Adults are attracted to tender growth and continually chase the young leaves at the top of the plant to lay eggs as the plant grows, so the distribution of citrus whitefly stages on a citrus plant from top to bottom is:

newly laid green eggs, blackened eggs, early-instar nymphs, late-instar nymphs, pseudopupae, newly emerged adults. The eggs are inserted into the leaf tissue through the stomata by an egg stalk and are extremely difficult to dislodge. The insect's development is very uneven, with overlapping generations; within 3 days after hatching, nymphs can crawl short distances on the underside of leaves, and once the mouthparts are inserted into the leaf tissue, they begin a sessile life, with dozens to hundreds per leaf at peak, preferring shady and humid environments. Peak adult periods occur in early to mid-May, early to mid-July, early to mid-August, early to mid-October, and late March to early April of the following year. Peak early-instar nymph periods occur in early to mid-March, early to mid-April, early to mid-June, and early to mid-September; the peak early-instar nymph period is the key time for chemical control.

2. Causes of outbreaks

First, no pest forecasting mechanism has been established, with blind spraying, especially frequent use of broad-spectrum insecticides, killing large numbers of natural enemies; at the same time, citrus growers lack understanding of the pest's occurrence patterns, missing the optimal control period, and spraying is not thorough, only spraying the upper leaf surface while neglecting the underside; second, the tree canopy is tall and dense, making spraying difficult and leading to missed sprays and untreated areas; third, the winter freeze damage of 1999 severely affected citrus orchards in the counties (districts) of northwestern Fujian, discouraging many citrus growers, and combined with low fruit prices in recent years, some orchards have been neglected or even abandoned; fourth, the serious mixing of multiple fruit tree species also creates certain difficulties for control.

3. Control methods

Adopt integrated management methods, based on agricultural control, actively applying biological control techniques, and according to the pest's occurrence patterns, using chemical, biological, and physical control measures to economically, safely, effectively, and simply control the occurrence of this pest.

First, biological control: first protect and utilize natural enemies, such as: Aschersonia fungi, ladybugs, lacewings, flower bugs, and parasitic wasps; when the pest is severe, the parasitic wasp Encarsia formosa (also known as the whitefly even-fan aphid wasp Encarsia formosa) can be artificially reared and released.

Second, physical control: this pest is attracted to yellow, so yellow boards can be set up to trap and kill adults. Method: use waste fiberboard or cardboard, cut into strips 1 m × 0.2 m, paint with orange-yellow paint, then coat with a layer of sticky oil (can use No. 10 machine oil mixed with a little butter), set 40-50 pieces per mu, placed between rows at a height comparable to the plants. When the whiteflies cover the board surface, re-coat with sticky oil promptly, generally re-coating once every 7-10 days. Yellow board trapping and release of Encarsia formosa can be coordinated.

Third, chemical control: because the whitefly has overlapping generations, chemical control is difficult, and there is currently no insecticide effective against all life stages; therefore, when using chemical control, spraying must be done several times in succession, focusing on the control of first and second generation early-instar nymphs in early to mid-March and early to mid-April. Optional insecticides include 25% Actara water-dispersible granules at 7500 times dilution mixed with 2.5% Kung Fu emulsifiable concentrate at 1500 times dilution, or 50% Kulong at 1500 times dilution mixed with 2.5% Kung Fu emulsifiable concentrate at 1500 times dilution, which have special efficacy against the whitefly; note to spray the orchard ground as well.

Fourth, agricultural control: rational pruning and shaping, thinning out overly dense orchards, and for mature plants with too many main branches, sawing off excess main branches and thinning out overly dense side branches before bud break in early spring; summer pruning should first remove overly dense vigorous branches and upright branches, keeping moderate and slanting branches to facilitate ventilation and light penetration in the canopy and reduce damage. At the same time, strengthen cultivation management, adopting the bud-thinning and shoot-releasing measure of "removing scattered shoots and keeping uniform ones, removing early shoots and keeping synchronized ones, concentrating shoot release" to break the whitefly's food chain, thereby reducing pest sources, lessening damage, and reducing the number of sprays. In addition, pay attention to orchard drainage, increase phosphorus and potassium fertilizer application, reduce the frequency of foliar nitrogen spraying, and improve tree resistance to pests and diseases. Furthermore, because the citrus whitefly reproduces rapidly and has migratory ability, it spreads easily; orchards within a certain area should carry out coordinated control to improve overall control effectiveness.