CITRUS AND SUBTROPICAL FRUITS

 Presented via Trans Texas Videoconference Network Cross listed as:

Fall 2006 

 
ARTHROPOD PESTS

Dr. J. Victor French, Professor and Entomologist
Texas A&M University Citrus Center, Weslaco, TX

OBJECTIVES:

1. To acquaint the student with the insects and mites which commonly attack    subtropical trees — including their classification and life cycles.

2. To become familiar with damage to and symptoms of subtropical trees caused by insect attack.

3. To become familiar with biological and chemical pest controls.

The climatic requirements and the perennial nature of the subtropical tree, as well as the vegetation associated with it favor the occurrence of large numbers of arthropod species in the ecosystem of the grove or orchard. Approximately 875 species of insects and mites are know to feed on citrus and subtropicals in various parts of the world. Of these, 72 are considered major in some part of the world. Texas citrus is attacked by 65 insect pests and mites of which 12 species are considered major.

These arthropod pests include: mites, armored scales, scale-like insects, whiteflies, true flies (Diptera), and ants. They are presented here by taxonomic division.

I. Mites

Mites belong to the class Arachnida, order Acarina: they are not insects. Most species deposit eggs. A six legged larvae is hatched from the egg. After a period of feeding, the larvae molts and becomes a nymph with eight legs. The nymph feeds and molds one or more times before the adult stage is reached.
 
  A. Citrus Rust Mite [Phyllocoptruta oleivora (Ashmead), family Eriophyidae] is the number one citrus pest. Only 7 to 10 days may be needed for the development of a new generation under conditions of high humidity and temperatures above 80^oF. Infestation is usually greater on the east side of trees and on abaxial leaf surfaces. Damage may occur on leaves, green stems, and fruits. Infestation on the fruit may lead to russeting and diminished size. Fruit quality is rarely affected, but affected fruit are graded lower as fresh or processed fruit. The mite is wedged-shaped, wider at the head, with only four legs located near the mouth and two false legs at the posterior end. Numerous segments or transverse rows of minute spines or furrows giving the appearance of segments are found on the abdomen. Young are bright yellow in color, becoming darker with age. Yellow spherical eggs are deposited singly or in clusters.

B. Texas Citrus Mite [Eutetranychus banksi (McG.), family Tetranychidae] is another major pest. Its main damage is to leaves, but fruits may be infested as well. Heavy infestation is indicated by a graying of the leaves which lead to defoliation. Populations increase in warm, dry weather. Adults are 1/70 inch long (0.36 mm), yellow to dark green in color, with dark blotches on the back down each side. Greater numbers are found on the south side and top of the trees. Eggs are disc-shaped and are usually laid on either side of the midrib and branching veins.

C. Citrus Red Mite [Panonychus citri (McG.), family Tetranychidae] A long established pest of California and Florida citrus, this mite first invaded Texas in 1980. Larger than the Texas citrus mite, it is characterized by a globose red body with prominent tubercles from which arise reddish bristles. Eggs are spherical and bright red, and have a central stalk with fine threads radiating from its top to the leaf surface. Red mites feed mainly on foliage, causing chlorosis and defoliation. Damaging infestations can develop during mild winters.

D. False Spider Mite [Brevipalpus phoenicis (Geijskes) and B. Californicus (Banks), family Tenuipalpidae] may appear in orchards if not specifically treated for. They infest the whole tree starting on the inside canopy. The pest causes spotting (Leprosis or nail head rust) of fruits, especially on grapefruit. Adult mites are 1/100 inch (.26 mm) long, flattened and slow moving. Legs are pale red and body red to purplish in color.

E. Avocado Brown Mite [Oligonychus punicae (Hirst), family Tetranychidae] are found primarily on the upper leaf surface along the veins. They produce a brownish discoloration, which may eventually cover the entire leaf. Heaviest infestations are found along roads, where dust on the trees appears to favor mite infestations. Severe infestations can cause defoliation.
 

II. Scales and Scale-like Insects

These pests belong to the class Insecta, order Homoptera , families Diaspididae, Coccidae and Pseudococcidae. They are a major pest to plant crops. Males have complete metamorphosis; females gradual metamorphosis. Males begin much as females but pupate at the end of the developmental period. Adult males appear as small two-winged flies without mouth parts. Females are wingless and usually immobile. All are covered with a waxy or resinous material. There are three groups:  soft scales, armored scales, and mealybugs. Soft scales and mealybugs secrete honeydew. Unlike the other two, armored scales produce a hardened covering and lose their legs. The others may be immobile at some period but may move when under stress. A sooty, black fungus (Capnodium citri) grows on the honeydew.
 
 

A. Chaff Scale [Parlatoria pergandii (Comstock), family Diaspididae] is one of the two major armored scales in Texas. These affect the entire tree: by sucking plant sap, these scales cause leaf-drop, fruit drip, twig die-back, and failure of the fruit to mature. Females' armor is light straw colored; the body of the insect showing through the scale gives it a purple to greenish cast. Several faintly marked ridges radiate from the origin of the scale. The males are slightly smaller than the females; their armor is thin, white, and almost oblong in shape. Some may appear brownish or gray. The scale insect's body under the armor is purplish in color.

B. California Red Scale [Aonidiella aurantii (Maskell), family Diaspididae] is the other major armored scale found in Texas. These infest the entire tree but are most common on leaves and fruit. The insect beneath is bright yellow in color. The scale's armor has a reddish cast and is circular with a distinct central peak or nipple. The male is 1/4 the size of the females. Fruits may be already infested when less than 1 cm in diameter. Effects of infestation are the same as those of the Chaff Scale. California Red Scale also attacks olive trees.

C. Brown Soft Scale [Coccus hesperidum (Linnaeus), family Coccidae] is the most important unarmored scale in Texas. Adults are brown to pale yellow, mottled, oval, 1/8 to 1/16 inches (3.2-1.6 mm) long. Population increases may occur when parasitic wasps and other beneficials are reduced by the spraying of insecticides. This scale is unisexual and considered ovoviviparous with eggs hatching in 2 to 3 hours after deposit. New larvae are yellow and infest the terminal portions of the branches. Adults settle on twigs and form dense colonies. Fecundity is low (100 eggs per female) but the number of generations is high (4-6 in the subtropics, up to 10 in the tropics). Ants are often associated with this species feeding on the honeydew and driving off or killing beneficials.

D. Cottony Cushion Scale [Icerya purchasi (Maskell), family Margarodidae] Populations often build when beneficials are reduced. Heavy infestations leads to reduced tree vitality. The body of the female is reddish brown, and legs and antennae are black, but this color is often masked by mealy wax. The female is hermaphroditic, possessing both male and female reproductive organs, which is rather unique among insects. A waxy fluted ovisac, one on each female, in which over 1,000 eggs may be deposited, causes the tree to appear as if covered by snow. Unfertilized eggs, which are rare, produce functional males, fragile looking dark red with grayish wings.

E. Black Scale [Saissetia oleae (Olivier), family Coccidae] is the major olive pest in California, it is also a citrus pest in Texas, Florida and California. Adult females are brown when young and become black when mature which are 2 to 5 mm long and nearly hemispherical, with a plainly visible elevated ridge resembling the letter "H" on their back. Black scale eggs are 0.2 to 0.3 mm long and light colored when first laid, becoming pink after 2 or 3 days. A few days before hatching they become reddish orange color, and nymphal eyespots become visible within the eggs. The first-instar nymphs, called crawlers, are 0.3 to 0.4 mm long and pale yellow to light brown, with dark eyes. The nymph's first molt is in 3 to 8 weeks. The second instar nymphs are 1 to 1.3 mm, long and the "H" becomes distinct. The nymphs migrate from the leaves to twigs at this stage. Chemical control is more effective against these stages than the next stage which is the immature adult and the adult stage.

Monitoring consists of selecting 10 trees in each area. Count the number of adults on the terminal 18 inches (45 cm) of 10 branches on each tree. Sum the numbers of black scale in each 10-branch sample and divide by the number of branches sampled to determine the infestation level. There are four levels: light (0 to < 1 per branch), moderate (1 to 4), heavy (4 to 10), and severe (>10). Light and moderate levels usually do not cause damage, but heavy and severe infestations require attention. Thinning the trees and pruning to open the canopy plus dormant oil sprays are recommended.

 F. Other olive scales are Olive Scale, Latakia scale, and Greedy Scale.

G. Citrus Mealybug [Planococcus citri (Rissa), family Pseudococcidae] and related species tend to favor the stems and buttons of trees, especially grapefruit. They may also congregate where fruits touch one another. Attacked fruit change color earlier in the season, fruit may drop and productivity is lowered. They copiously excrete honeydew. The insect body is distinctly segmented, about 1/4 inch (0.64 cm) in length, covered with powdery wax, with the appearance of "having been rolled in flour". The females have antennae and legs, and are active throughout the linesman. They form an ovisac consisting of interwoven waxy fibers and resembling a light cottony mass, into which they deposit several hundred eggs (oviparous species) or first instar larvae (ovoviviparous species). The males form a whitish cottony cocoon.

H. Long-tailed Mealybug [Pseudococcus longispinus (Targioni-Tozzetti), family Pseudococcidae] a soft oval, flattened species attacking new scion growth on grafted avocados and citrus, destroying small tender foliage.


 III Whiteflies

Whiteflies belong to the class Insecta, order Homoptera, family Aleyrodidae. A complex of six different species attack Texas citrus including: citrus whitefly, Dialeurodes citri (Ashmead); cloudy-winged whitefly , Dialeurodes citrifolii (Morgan); woolly whitefly, Aleurothrixus floccosus (Maskell); Paraleurodes citri (no common name); sweet potato whitefly Bemisia tabaci (Gennadius); and the citrus blackfly Aleurocanthus woglumi (Ashby).

The adult whiteflies have their two pairs of wings covered with a white, powdery wax which gives the insects their common name (exception is the citrus blackfly).

 
A. Citrus Blackfly [Aleurocanthus woglumi (Ashby), family Aleyrodidae] which was introduced from Mexico causes the most serious damage to Texas citrus. The adult has slate-blue wings, red abdomen, reddish eyes, and 1-2 mm in length. Eggs are laid in a unusual spiral pattern on the leaf undersurface. This pest is best controlled biologically by two wasp parasite species — Encarsia opulenta and Amitus hesperidum. IV Flies True flies belong to the class Insecta, order Diptera, family Tephritidae. Fruit flies are among the most serious pests of citrus worldwide. Eggs are laid just below the fruit surface. A single female can lay as many as 800 eggs in a lifetime. The larvae (maggots) burrow through the fruit. When full grown the maggots drop to the ground and pupate in the soil, and later emerge as adult flies.  
A. Mediterranean Fruit Fly [Ceratitis capitata, (Wiedermann), family Tephritidae] found throughout subtropical regions of the world on a wide variety of decidious and tropical fruits including peaches, avocados, all citrus fruits except lemon and sour lime. Smaller than a housefly its wings are held in a drooping position. Black spots on the thorax, two white bands on a yellow abdomen and characteristic black and yellow marking on the wings identify this fly species.

B. Mexican Fruit Fly [Anastrepha ludens, (Loew), family Tephritidae] is sometimes a problem with late fruit. Adults begin migrating from Mexico during late December or January after which quarantine regulations go into effect for certain interstate shipments of fruit. There is some question as to whether the flies can survive under Valley summer weather conditions. Larvae cause breakdown of the fruit either on the tree or after harvest.
 

IV Flies IV Flies IV Flies V Ants
                               Ants belong to the class Insecta, order Hymenoptera, family Formicidae. As social insects, ant colonies are typically made up of soldiers, minor and major workers, legless larvae (brood), winged (alate) males, female reproductives an   one or more queens. Often vastly outnumbering other insects, ants cause considerable damage to citrus and other subtropical plants.

B. Tropical fire ant [Solenopsis geminata (Fabricius), family Formicidae] is a major citrus pest. These pugnacious ants inflict painful stings to orchard workers who step unsuspecting on their mounds (colonies) made up of loose soil on the orchard floor. Devastating to young citrus, fire ants feed undetected beneath the freeze protectant tree wraps, and girdle the trunks and introduce Phytophthora parasitica (foot rot fungus). Fire ants also attend aphids, mealybugs, and whiteflies for their 'honeydew' and kill or drive off beneficial insects and mites — thus, interfering with biological control.

C. Red imported fire ant [Solenopsis invicta Burren, family Formicidae] has recently invaded the Lower Rio Grande Valley and is beginning to replace the tropical fire ant in Texas citrus orchards.