Northern Nut Growers Association Report of the Proceedings at the 41st Annual Meeting Part 21

Insecticides for Nut Insects

E. H. SIEGLER United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine

Fortunately, the growers of nuts do not have to combat a large number of injurious insect species. However, some species do at times cause a heavy loss of nuts and may also damage the vegetation growth of the trees. Injury by insects will vary from year to year, due to various causes, and insects frequently show varietal host preferences. Timely use of insecticides is the most effective means of combating many harmful species.

Until the beginning of World War II a rather limited number of insecticides was available, such as lead a.r.s.enate, cryolite, nicotine, mineral oil emulsions, and rotenone. Some injurious insects were satisfactorily controlled through the timely application of one or the other of these materials, or combinations of them; others survived in damaging numbers in spite of all attempts to suppress them.

During and since World War II, both in the United States and abroad, work on insecticides has been stepped up markedly. As a result, many new insecticides have been developed and are available for general use.

The first of the new insecticides about which we heard was DDT.

Actually, the compound itself was not a new one, since it was prepared by a German student chemist in 1874. However, no use was found for it until 1939, when a Swiss chemist found it promising as an insecticide against the Colorado potato beetle. It was first tested in the United States a few years later.

Since the successful introduction of DDT, promising new insecticides have become available more frequently and in greater numbers than ever before. Among these materials are certain chlorinated hydrocarbons related to DDT. These include methoxychlor and TDE, neither of which is, on the whole, as useful as DDT but both of which are of value and have an important advantage over DDT in that they are reported to be less toxic to warm-blooded animals. Other new chlorinated hydrocarbons include benzene hexachloride, synthesized in 1828 and first tested against insects in France in 1941 and discovered about the same time in England; chlordane, developed in the United States a few years ago; and toxaphene. Several organic phosphorus compounds, including hexaethyl tetraphosphate, tetraethyl pyrophosphate, and parathion, have also been developed.

Technical benzene hexachloride is a mixture of several isomers, the gamma isomer being the most toxic to insects. The practically pure isomer is known as lindane. A handicap to the general use of benzene hexachloride on fruit is its tendency to cause off-flavor condition when applied too close to harvest. Lindane is less likely to cause off-flavor in fruit than technical benzene hexachloride but may not overcome this fault altogether.

The organic phosphate insecticides, like DDT, were first found of value in Europe and were introduced into the United States after the close of World War II. Parathion in particular shows great promise for the control of many insect pests. Although these compounds are very poisonous and must be handled strictly in accordance with the manufacturers' recommendations, a recent announcement by Arnold J.

Lehman, of the Food and Drug Administration, indicates that their residues are not likely to be harmful. He has stated that "parathion is not stored in the tissues to an appreciable extent--it is rapidly destroyed by the tissues of the body which in turn is an added mechanism for the prevention of tissue acc.u.mulation." Residues of hexaethyl tetraphosphate and tetraethyl pyrophosphate persist for only a short time and residues of parathion drop to a low level within 10 to 14 days after application. This information, however, does not make it unnecessary for the user to observe strictly all warnings and precautions issued by the manufacturers of parathion and of other organic phosphates. Serious effects and deaths have occurred though excessive exposures to parathion.

General Information Regarding the Use of the New Organic Insecticides

~Handling the insecticides.~ All the new organic insecticides, the organic phosphates in particular, are to some degree toxic not only to many insects but to man and animals as well. Even the most toxic ones can be used, however, without harmful effects on the operator, provided all the cautions issued by the manufacturer are properly followed. Special care must be taken in handling concentrated insecticides preparatory to making diluted spray or dust applications.

~Spray concentrations.~ DDT has been used more extensively than any of the other newer insecticides and for this reason there is considerable information relative to the spray concentrations known to be effective against insects susceptible to it. For spray purposes DDT is generally employed at the rate of 1-1/2 to 4 pounds of 50 percent wettable powder per 100 gallons of water.

Parathion is being used at 1/2 to 1-1/2 pounds of 15 percent wettable powder per 100 gallons of water for mites and up to 2 pounds to 100 gallons of water for insects more resistant to it. The occurrence of injury to the foliage and fruit of some varieties of apples when this insecticide is used is under investigation.

Benzene hexachloride (10 percent gamma isomer, wettable) is being used at 2 to 4 pounds, and sometimes less depending upon the insect, per 100 gallons of water. Wettable mixtures containing 25 percent of lindane (approximately pure gamma isomer) are used at dosages which would give an equivalent quant.i.ty of the gamma isomer in the diluted spray.

Chlordane is usually employed at the rate of 2 to 3 pounds of 50 percent wettable powder and toxaphene at 2 to 4 pounds of 40 percent wettable powder per 100 gallons.

These insecticides are also being sold for use as dusts, either ready to use or in a more concentrated form which can be reduced to dusting strength through the addition of inert material.

~Spray Residues.~ Spray residues are not important on nut crops, but on fruits it is important to time the insecticide applications so that harmful residues are avoided. Animals should not be allowed to graze vegetation beneath trees recently treated. Instructions on the packaged insecticide should be followed.

~Effect on beneficial insects.~ Since the more potent of the newer organic insecticides are toxic to many parasitic and predatory insects, all of which help to reduce the populations of injurious species, these insecticides, if used, must be largely relied upon to effect control by themselves. Often no immediate a.s.sistance is forthcoming from beneficial insects after these materials have been used.

Nut Insect Investigations

Except for studies on the chestnut weevils, nut insect investigations by the Bureau of Entomology and Plant Quarantine are being conducted primarily on the pecan at southern laboratories. Many of the remarks in this paper are therefore based on information obtained from these laboratories. In view of the short time the new organic insecticides have been available, work to determine their place in nut insect control programs is largely in the experimental stage. Much further work will be necessary before detailed instructions can be given for their general use.

Insects Attacking the Nuts

~The Pecan weevil.~ The adult of the pecan weevil[8] is a snout beetle that attacks not only pecan throughout the South but also hickory in the eastern half of the United States. During mid-season, previous to the formation of the kernel, nuts are frequently punctured for feeding purposes. This results in failure of the nuts to complete their development. The princ.i.p.al injury, however, is caused by grubs that develop from eggs laid in the nuts after the kernels have formed. This is usually during September on pecans in the South. The grubs feed on the kernels and may consume them completely (Fig. 1).

[8] ~Curculio caryae~ (Horn).

Applications of sprays containing 6 pounds of 50 percent DDT wettable powder per 100 gallons of water just previous to and during the oviposition period have proved effective against this pest.

[Ill.u.s.tration: Fig 1.--Nut infested with larvae of the pecan weevil.]

[Ill.u.s.tration: Fig. 2.--Larva of the b.u.t.ternut curculio in j.a.panese walnut shoot.]

[Ill.u.s.tration: Fig. 3--Adults of the walnut husk maggot on black walnut.

Enlarged.]

[Ill.u.s.tration: Fig. 4.--Adult of a leaf-footed bug. Enlarged.]

[Ill.u.s.tration: Fig. 5.--Defoliation caused by the black pecan aphid.]

Nut curculios. Several species of curculios, such as the b.u.t.ternut curculio[9] (Fig. 2) and the hickorynut curculio,[10] infest the fruit of these and other nut trees. Their life histories and methods of attack are somewhat alike and for the purpose of this report the b.u.t.ternut curiculio is given as an example. This insect lays its eggs in both the young shoots and nuts, which usually drop as a result of the injury.

The larvae then develop to maturity within the dying tissues after which they enter the soil and transform to adults. Subsequently they leave the soil to pa.s.s the winter above ground protected from low temperatures by weeds or other vegetation.

[9] ~Conotrachelus juglandis~ Lee.

[10] ~Conotrachelus affinis~ Boh.

Lead a.r.s.enate, 4 pounds per 100 gallons of water, has been relied upon in the past for control of various nut curculios. Among the newer insecticides, benzene hexachloride (6 percent gamma), 4 to 6 pounds per 100 gallons, has shown promise against a shoot curculio on pecans when applied soon after the trees start growth in the spring.

~Hickory shuckworm.~ The hickory shuckworm[11] is another serious pest of pecan and hickory nuts. Early in the year, previous to the hardening of the sh.e.l.ls, the kernels are eaten. This injury causes many of the nuts to drop. In the fall, the later generations tunnel within and feed upon the shucks only. The affected nuts are usually smaller than normal; in addition the sh.e.l.ls are often stained and are more difficult to separate from the husks.

[11] ~Laspeyresia caryana~ (Fitch).

Extensive experimentation in the control of this insect has been carried out without much success. No effective insecticide treatment can be recommended for its control.

~Walnut husk maggot.~ The adult of the walnut husk maggot[12] is a fly (Fig. 3); it is related to other injurious fruit flies such as the apple maggot, Mediterranean fruit fly, and the oriental fruit fly, which has recently been found in Hawaii. Adults emerge from the soil and fly to the trees in midsummer. Egg laying follows in 1 to 3 weeks, the eggs being deposited on the husks of several kinds of nuts. The maggots feed within the husks. Not only is the quality of infested nuts lowered, but, in addition, the husks are more difficult to remove. A closely related species is particularly damaging to the Persian or English walnut in California.

[12] ~Rhagoletis suavis~ Loew.

Lead a.r.s.enate, 2 to 4 pounds per 100 gallons of water, in combination with an equal quant.i.ty of hydrated lime is quite effective in destroying the adults of the walnut husk maggot when applied at the time they are present.

~Stinkbugs and leaf-footed bugs.~ There are a number of stinkbugs and leaf-footed bugs (Fig. 4), in addition to the species mentioned,[13][14]

which are responsible for important injuries to pecans, filberts, and other nuts. These insects puncture the immature nuts with their beaks.

The punctured areas become spongy, somewhat dark in color, and are bitter to the taste; on pecan the typical injury is referred to as black pit and kernel spot.

[13] ~Nezara vizidula~ (L.).

[14] ~Leptoglossus phyllopus~ (L.).

Crops of favorable host plants such as cowpeas and soybeans should not be planted in or adjacent to nut orchards subject to attack by these sucking bugs. In general, orchard sanitation should be practiced.

[Ill.u.s.tration: Fig. 6.--Galls produced by the pecan phylloxera.]