In the early stages of many horticultural industries insect pests and fungous diseases are not troublesome, but as the industry develops its enemies become more numerous. So it has been with the avocado. During the first few years in which this fruit was planted commercially in Florida little injury was caused by parasites, but recently it has been necessary to combat vigorously the insects which prey on the tree, and also several fungous diseases.
In California the avocado has, up to the present, been comparatively free from the attacks of insect and fungous pests; yet several insects have made their appearance in the orchards and must be watched carefully lest they become so numerous as to cause serious harm.
Thrips and red-spider are the most common insects which attack the avocado in Florida. Red-banded thrips (Heliothrips rubrocinctus Giard.) and the greenhouse thrips (Heliothrips haemorrhoidalis Bouche) feed on the foliage, sometimes causing much damage. Both these species are exceedingly small, soft-bodied, fringed-winged insects, with piercing mouth-parts by means of which they puncture the epidermis and extract the juices from the leaves. They are most destructive in early spring, their numbers being greatly reduced when the summer rains commence. Spraying with nicotine solutions has been quite effective in controlling them.
The red-spider (Tetranychus mytilaspidis Riley) also does considerable damage during the spring months. This insect, which is scarcely larger than a pin point, can be detected on the foliage without the aid of a magnifying glass because of its bright red color. It feeds on the avocado by piercing the leaf tissues and extracting the plant juices. Often it becomes so abundant as to cause the leaves to assume a brownish, sickly appearance. It occurs commonly in California as well as in Florida, but has not yet been reported as attacking avocados in California. Lime-sulfur mixtures have been used successfully in combating this insect. For citrus trees, H. L. Quayle recommends commercial lime-sulfur, dry sulfur and hydrated lime, and distillate emulsion. These may all prove to be effective with the avocado as well.
Among the scale insects which commonly attack the avocado, the most important are the black scale (Saissetia oleoe Bern.), and a soft white scale (Pulvinaria pyriformis Ckll.), the latter being a serious pest in Florida. Severe infestations of the black scale are occasionally found on old seedling trees in California, but this insect has not yet become a pest in the young avocado groves of that state. The wax scale (Ceroplastes floridensis Comst.) is occasionally found on avocados in Florida, but rarely requires combative measures. All of these scale insects, as well as a white fly (Trialeurodes floridensis Quaint.), which has become troublesome on some of the Florida Keys, can probably be controlled by the use of oil sprays.
The citrus mealy-bug (Pseudococcus citri Risso) has been reported on the avocado in Ventura County, California, but it is not known to have caused extensive damage. The avocado mealy-bug (Pseudococcus nipoe Mask.), which is a serious pest in Hawaii, has been found in southern Florida groves. It sometimes becomes very troublesome. D. F. Fullaway of Hawaii recommends that it be controlled by spraying with oil-emulsions.
The presence of the avocado weevil (Heilipus lauri Boh.) in California, where it was probably introduced from Mexico in avocado seeds, caused the Federal Horticultural Board to prohibit the importation of seeds of the Mexican race from Mexico and Central America. This insect is a small black beetle which tunnels in the seeds, and is said to do considerable damage.
Other seed weevils attack the avocado in various parts of the tropics. H. S. Barber describes the more important ones, so far as they are known, in the Proceedings of the Entomological Society of Washington, March, 1919. Heilipus pittieri Barber, from Costa Rica, is similar to H. lauri. Conotrachelus perseoe Barber does great damage to avocados in Guatemala. Its larvae have been found in avocado seeds sent to the United States, but it is believed the species has not become established in this country. Once thoroughly established, the seed weevils are difficult to exterminate, hence it is to be hoped that they will not gain a foothold in this country.
In Guatemala, Trioza koebelei Kirkaldy (and perhaps other species) produces leaf-galls on the avocado, often in such great numbers as seriously to affect the health of the tree.
In addition to these insects, a number of others have been reported as attacking the avocado in various parts of the tropics. These include numerous scale insects, both armored and un-armored, several borers, and the well-known Mediterranean fruit-fly (Ceratitis capitata Wied.); the better-known species are listed in the Manual of Dangerous Insects published by the Department of Agriculture.
In the dry climate of California, fungous parasites give the avocado grower comparatively little trouble, but in Florida and in many parts of the tropics they may require stringent combative measures.
The following extracts from a paper by H. E. Stevens, published in the Proceedings of the Florida State Horticultural Society for 1918, cover the situation as regards fungous pests in Florida as it exists at the present time:
"Leaves and frequently fruits of the avocado are attacked by a fungus which is probably a species of Gloeosporium. The affected leaf is usually attacked at the tip, and the disease gradually spreads until the greater part of the blade is involved, when the leaf falls. Severe attacks may cause considerable defoliation of trees and result in the death of young terminal twigs. Fruits may be attacked when small, in which case severe shedding may follow. If the more mature fruits are attacked, a brown spotting is produced and the skin may crack.
"Another common type of injury, frequently noted on the fruits, is referred to as anthracnose by some of the growers. This type of injury is very similar to melanose of citrus fruits in general appearance. It is superficial and appears in the form of dark reddish brown caked masses on the surface of affected fruits. The markings are hard, compact, and the surface is cracked or broken. The injury may-cover only a part or the whole surface of the fruit. It makes an unsightly fruit, but apparently does not affect the quality. The disease is apparently caused by a fungus, perhaps a Gloeosporium or a closely related species.