The egg hatches into a larva, or "worm," which bores into the fruit. It becomes full-grown in from twelve to twenty days and bores out of the fruit. It enters the soil, burrows to a depth of one-half to two inches, and forms an earthern cell in which to pupate. In three or four weeks it emerges as a full grown beetle and attacks the ungathered fruit and the foliage. On the approach of cold weather the beetle seeks a protected place in which to pass the winter.
The character of the injury is very nearly alike in all fruits. In the plum the fruit often falls to the ground before mature. In seasons of short crops very little fruit may remain to ripen. The punctures cause the fruit to become mis-shaped and to exude masses of gum. The ripe fruit becomes "wormy." The late varieties may be seriously injured by the new generation of adults. In the apple the injury to the fruit is about the same as in the plum, except that the infested fruit is not so likely to fall to the ground and that the egg rarely hatches into the grub there. The fruit becomes knotted and pitted. The late varieties may also be injured by the new generation of adults. In the peach, cherry and other stone fruits, the injury closely resembles that of the plum.
Although the plum curculio has some natural enemies that tend to reduce its numbers somewhat, yet they are not important enough to be considered as effective means of control. Some of these natural enemies are parasites of various kinds, birds, chickens and the like.
There are several remedial measures practiced, varying in their degree of effectiveness. Away back in the early days of horticulture in this country, when the curculio became very abundant rewards were offered for an effective method of combating it. Several were proposed, but only a few were at all effective. The best of these methods is what is called "jarring."
The curculio has the habit of falling to the ground and "playing 'possum" when disturbed. This led to the practice of holding or spreading sheets beneath the tree and then striking the tree a sudden, forcible blow with a padded pole or mallet in order to dislodge the beetles. The trees were jarred daily from the time the calyx or "shuck" began to slip from the newly set fruit until the beetles had disappeared, or for at least four or five weeks. This was practiced to quite an extent, but it takes too much time and is too expensive.
A still better remedy is clean cultivation. Experiments have shown that as high as 76.75 per cent. of the pupae may be destroyed by means of thorough cultivation. The mere breaking of the pupal cell, leaving the earth in contact with the body of the pupa, is fatal to many. Others are killed by the crushing action of the earth as it is stirred. Others are exposed to the elements and subject to the attacks of their enemies, such as ants and birds. Sunlight is quickly fatal to them, and exposure to the air on a warm day in the shade is also fatal to them. Observations show that the insect is in the pupal condition in the ground in from fifty to sixty-five days after the falling of the blossoms of such fruit as apples and plums. Data have been presented to show that the minimum time spent in the ground is about twenty days. Shallow cultivation should begin, therefore, in about eight or nine weeks after blossoming. It is best to cultivate every week or oftener for six or seven weeks. It is very necessary that this cultivation should reach immediately beneath the spread of the limbs, as most of the curculios are found here, having dropped from the fruit above and burrowed into the soil where they fell.
The third method of combating the curculio, the method most commonly used and most generally recommended, is spraying with arsenical poisons. The spray most generally used is arsenate of lead. The most economical and effective way is to add arsenate of lead to Bordeaux mixture. The Bordeaux is mixed in the following proportions: three pounds of copper sulphate (blue vitriol), four pounds of lime, and fifty gallons of water. To this amount of Bordeaux mixture three pounds of arsenate of lead are added. In place of Bordeaux mixture lime-sulphur may be used. If the insecticide is used alone, three pounds of arsenate of lead in fifty gallons of water make an effective spray. It is best to spray three times, the first spraying coming just before the blossoms open, the second coming ten days later, and the third another ten days later. The cost is from ten to fifteen cents per tree for the three sprayings. This cost is lessened when combined with other sprays.
While spraying greatly reduces the injuries inflicted, yet it is apparent that account must be taken of other factors, such as the relative abundance of insects as compared with the amount of fruit present on the trees. With a small fruit crop and an abundance of curculios, the most thorough spraying in the world will not serve to bring through a satisfactory amount of sound fruit.
While spraying is undoubtedly the most important aid and, if persisted in from year to year, may answer for its control, as its effects are cumulative, yet it is clear that other control measures should also be employed. In all cases which have come under observation the insects have always been found most abundant in orchards which are in sod or are poorly cared for and allowed to grow up more or less in weeds and trash. Also, orchards near woods always suffer severely, especially along the border. As opposed to this condition is the notably less injury in orchards kept free from weeds and trash. In such cases spraying usually given for other insects, as the codling moth, serves to keep the curculio well under control. In fact, it may be said as a general statement that the curculio will never become seriously troublesome in orchards given the usual routine attention in cultivation, spraying and pruning now considered essential in successful fruit growing. Serious losses from the curculio are almost conclusive evidence of neglect, which is best and most quickly corrected by the adoption of proper orchard practice.