Our 250-acre family farm has about 100 acres of certified organic apple trees of various varieties, sizes, and ages (from fifty years to one year). They all have one thing in common – an exceptionally high percentage of the apple trees’ spring fruit gets injured by the dreaded plum curculio (PC) beetle. For ten years, we have been battling this beetle in our orchards with no success. Anti-feeding agents, like garlic, help only minimally. Irritants like diatomaceous earth help only minimally, kaolin clay helps minimally, and there are no natural predators to help reduce the residential population. Even with all of the known effective cultural practices, overall apple damage ranges from 10 to 50 percent. The plum curculio problem is universal throughout all apple-growing areas through the eastern United States, not only in organic orchards, but also in non-organic orchards that have chosen to eliminate nerve poison pesticide in their spray program. Much time and a lot of money has been spent trying to eliminate the fertile female beetle that marks up 1,000 fruit while only laying eggs in 250 of them. This all occurs during the first couple of weeks after the fruit first begins to grow.
The apples that contain a PC egg are protected from predation by other insects. The egg hatches into a worm that feeds within the growing apple. Mother Nature’s plan is that the apple is aborted by the tree in late June, where if falls to the ground. There, it is hidden and sheltered in the orchard grass. After a couple of weeks, the larva exits the apple, enters the soil and pupates down about 1 to 2 inches. It then emerges in mid-August as an adult beetle. It forages on apples and vegetation in the orchard all fall and winter long, and then, after spring mating, the egg-laying cycle begins once again.
My research project focused on trying to destroy the larvae within the apple (“June drop apples”) before they exited the fruit to pupate within the soil. Hundreds of June drop apples lie on the ground under the tree, each containing a female with the potential to blemish another 100 apples the following year. It seemed that the quickest, most efficient means of removing the apples under the tree and destroying this fruit was to sweep them into the center drive lane of the orchard and then chop/crush the little apples. I believed that the apples could be removed from under the tree using a Phil Brown Brush-sweeper, if the angle of the head was modified and the paddle brush gripper teeth were removed. The equipment was purchased, retrofitted to my John Deere 2240 with the appropriate modifications, and the speed of the tractor and sweeper head rotation were coordinated.
By July 5th of 2004, most of the infected apples had fallen to the ground. Under some of the rows of trees in a 30-year-old orchard of 16 foot x 20 foot planting, I swept the trees twice (June 26th and July 5th). Other rows of trees were swept only once. This was to determine if intervals between sweepings affected the effectiveness of the machine. There was no difference.
Prior to sweeping, several test trees were chosen and small marking stakes were driven into the ground with a slight protrusion to mark out 3 foot x 3 foot test areas. The apples were counted before and after sweeping in these chosen plots to determine the percentage of June drop apples that were removed by sweeping from under the trees. Two passes made under each tree, driving at 3 mph (which was determined to be the optimum speed) removed 94 percent of the dropped apples. This was better results than I had hoped for. It appeared that the only apples left under the tree were old, small, spongy apples that may not have even contained any PC larvae. The first half of the project appeared to be very successful.
The second half of the project didn’t fare quite as well. The year 2004 was a record-breaking spring and summer (for rain). The June drop apples, when I tried to flail-mow them and crush them in the dry valleys, were simply pushed into the moist sod without actually being crushed. I had hoped that the sweeping action had bruised the fruit enough so that it decayed faster than the PC larvae could use it for its development in its last larvae instars. I took apple damage counts (12 percent) on the remaining apples on the trees and decided to try this in another year in order to compare damage counts on the fruit. This would serve as an indication of whether or not I had disrupted the next generation of PC.
This year’s spring and early summer were more normal with perfect soil conditions to crush the apples. The orchards were swept, but with the drier conditions, this became a dirty job. Dust and pulverized leaves mandated that I wear goggles and a dust mask as important pieces of protection. Every few rows, the tractor’s radiator needed to be washed off and blown out. This was a nasty work situation and it quickly became apparent that this was not a practical way to move the apples into the drive lane.
When I tried to crush the ½ inch apples in this dry sod, they were still not being destroyed. Unnoticeable slight indentations in the soil’s surface gathered these small apples, and then protected the apples from being crushed as the roller went over them. Only the apples on a totally flat, table-type surface were damaged by the flail mower’s roller. This, in older established orchards where the dry valleys were slightly rutted by tractor traffic through the years, amounted to a destruction of June drop apples of only 20 percent. I felt that this number wasn’t significant enough for all of the time and mess, and would not be considered an effective cultural tool to help reduce PC numbers. There are too many slight imperfections in the seemingly flat dry valleys within the orchards, which serve to protect most of the apples from being destroyed.
I did an apple damage count again this fall, and found only a slight reduction of PC damaged fruit compared to the previous year (10 percent). This could probably be attributed to population due to seasonal weather effects, and not cultural practice.
As of 2005, no one has found a way to control PC in organic orchards anywhere in the United States. These beetles can only complete their life cycle if the larvae can get the fruit to drop to the ground at the right time so that the larvae can effectively exit the fruit and go into the ground and pupate, later emerging as adults. The sweeper did a good job of removing trash and perennial weeds from under the trees, as well as moving the apples to the dry valley. In addition, early morning sweeping of damp leaves, or sweeping during or right after a rain eliminates most of the dust and leaf debris from floating in the air. The unresolved problem exists in destroying the larvae and/or its home. Possibly a roller mechanism with little spikes or an irregularly-surfaced roller would overcome the slight peaks and valleys of the soil surface in the drive lane that protect the June drop fruit. I fee that the key to PC control in organic orchards will be destroying or eliminating the June drop apples from the orchard in a timely fashion. This investigation needs to continued by others.