IMMY J.WELCH and HORACE W.VAN CLEAVE*
THE HICKORY SHUCKWORM, Laspeyresia caryana (Fitch), is one of the most serious pecan pests in Texas. Inasmuch as the biology of this insect is poorly known, many difficulties have been encountered in attempting to control this species. Such habits as the nearly continuous presence of adults during the growing season caused by overlap in generations, and the habit of the larvae mining the shucks of the pecans, which minimizes the duration of contact with insecticidal applications, etc., present a number of obstacles. For these reasons, the biology of the hickory shuckworm was studied to find what period in the season they caused the most damage, and to find which stage in their development that this insect could be controlled most easily.
Field Observations
During 1966 the biology of the hickory shuckworm was studied by the use of light traps, field observations and collections.
To obtain information on adult shuckworm emergence from the over-winter-ing generation, pecan shucks were collected from various sites in the state during the late fall of 1965, and held in emergence cages at College Station. The following spring checks were made at 48-hour intervals, and emerging adults were collected and their numbers were recorded.
The following generations of hickory shuckworm were studied primarily in the field by observing the insect on various species of host plants which included pecan, Carya illinoensis, bitter pecan, Carya aquatica and black hictory, Carya texana. Data were collected on the approximate times of adult shuckworm emergence from the pupal stage for each generation. Duration of the four major stages of development of this pest was observed for each generation. Light traps were used also to indicate the presence of adults in an attempt to estimate the population densities in the field.
The hickory shuckworm passed the winter as mature larvae in the shucks of pecan and hickory on the ground or on the tree, and emerged as adults in the spring. Adults from the overwintering generation emerged as early as February 16, with the maximum emergence between April 5 and May 10, in the College Station area. These adults laid their small, whitish, oval eggs on hickory and pecan trees. Most of these first generation larvae were found feeding on phylloxera galls of pecans and hickory, small hickory nuts and tender shoots of pecan.
The second generation of this pest was found in the same situations as the first, with the exception that some larvae began to feed on small pecan nuts. As a result of this nut feeding, affected nuts were shed. This results in a high rate of mortality among the larvae of this generation; however, some of the larvae were able to survive within these fallen nuts and complete their life cycle. During 1966, the pecans mined by the second generation larva appeared to be so small that only a small percentage completed their development; however, during 1967 a higher percentage survived, apparently due to an increase in size and development of the nuts during the corresponding period of the year.
Coinciding with the third generation, the pecan nuts in this area appeared to be large enough to allow full larval growth and the shuckworm populations on pecans increased with each succeeding generation. This pest reached its highest population levels from late August until harvest. During this period, the larvae feed on the shuck which often results in incompletely filled nuts and pre-vents the natural separation of the shuck from the nut. Nuts having shucks infested by the shuckworm late in the season usually have a stained shell which may lower the market value of in-shell pecans.
A summary of the approximate time of the maximum occurrence of the adults of the hickory shuckworm for each generation and the number of generations of the shuckworm in the College Station area is shown in figure 1. Due to the overlap of most generations, adults of this pest could be found in the test area almost daily throughout the pecan growing season.
Laboratory Studies
The hickory shuckworm undergoes four stages of development: egg, larvae, pupa, and adult. Each of these stages was studied in the laboratory at 80⁰ F. with a relative humidity of 40 to 50 percent, to compare the biology in the field with that in the laboratory.
During 1967, the hickory shuckworm was reared through three generations in the laboratory on a wheat germ medium used by Adkisson et al. (1) for rearing the pink bollworm, Pectinophora gossypiella (Sanders). The diet consisted of wheat germ, vitamin mixture and water. The ingredients were blended and cooled to 60⁰ C., and poured into plastic shell vials 11/16”x2-1/2”to a depth of about 3/4 inch. The vials filled with medium were allowed to set at room temperature for about a day to evaporate free moisture before confining larvae in the vials.
Eggs were obtained both by placing adult shuckworms in screen mesh sleeve cages over nut clusters and leaves on pecan trees, and by placing adult moths in 1-gallon ice cream cartons containing nut clusters and pecan leaves. A small vial containing a 10 percent sugar-water solution was placed in each 1-gallon carton to serve as a source of food for the moths. A small cluster of pecan nuts and leaves were placed in the carton for an oviposition site. The eggs were held until hatching, and then the newly hatched larvae were transferred to the shell vials containing the artificial diet. The vials were then plugged with sterilized nonabsorbent cotton and placed in a bioclimatic chamber under a daily light-dark period of 15:9 and at SO⁰ F. and 40 to 50 percent relative humidity. The larvae were observed daily and information on the number of molts per larvae, duration of each larval instar, and the pupal stage was obtained. Information on pre oviposition period, oviposition period, and number of eggs lay per female and longevity of the adults was obtained by observing the adults reared under laboratory conditions.
The life cycle of the hickory shuckworm reared in the laboratory on artificial medium is given in Table 1. The eggs of this insect hatched in an average of 4.13 days. The duration of the larval period was from 19 to 49 days, with an average of 33 days. During this period, the larvae went through six or seven instars. The pupal period had an average duration of 9.11 days. The data involving pre oviposition, oviposition and the number of eggs per female were obtained from the mating of four individual pairs of adults in which the males and females were of the same age. The average length of the pre oviposition and oviposition period is 6.43 and 4.74 days, respectively. The observed moths laid from 25 to 153 eggs, with an average of 73.24 per female. The total life cycle of the hickory shuckworm from egg to egg was 37 to 69 days, with an average of 52.73 days.
On the basis of the material examined, the laboratory data presented corresponded closely to what was noted in the field; field and laboratory reared generations coincided, specimens were of comparable size, etc.
Table 1.Life cycle of the hickory shuckworm on an artificial diet (wheat germ) in the laboratory (80⁰ F.: 40 to 50 percent relative humidity: light, 15 hr.; dark, 9 hr.)
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Duration in days
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Minimum Maximum Average
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Incubation period ……………………………. 4 5 4.13
Larval period ……………………………………. 19 49 33.00
Instar 1 ……………………….……….. 2 5 4.05
Instar 2 …………………..……..…….. 1 4 2.67
Instar 3 ………………………….…….. 2 6 3.17
Instar 4 …………………………….….. 1 7 2.83
Instar 5 …………………………….….. 2 4 2.89
Instar 6 ………………………………… 7 (2) 1/ 28 (6) 1/ 13.55 (3.89) 1/
Instar 7...................................... 8 30 17.33
Pupal period …………………………………….. 7 13 9.11
Pre oviposition period ………………………. 5 7 6.43
Oviposition period ……………………………. 2 8 4.74
No. eggs per female ………………………….. 25 153 73.24
Percentage of eggs hatching ……………… 76.00 88.28 83.65
Longevity of adult males …………………… 6 20 10.60
Longevity of adult females ………………… 9 17 14.17
Duration from egg to adult ……………….. 28 57 42.17
Total life cycle (from egg to egg) ……..... 37 69 52.73
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1/ Fifty percent of the test larvae pupated after six instars while the remainder pupated after seven instars. The numbers enclosed within the parenthesis represents the duration of the sixth instar for those larvae passing through seven instars prior to pupation. No morphological differences were noted between these larvae prior to the sixth instar.
Control
Osburn et al. (2) recommended EPN and Guthion® for late-season control of the hickory shuckworm. These recommendations called for three applications, the first about August 10 to 15, and the second and third at 2-week intervals.
Little work on control of this insect has been done in Texas, and the Texas
Agricultural Experiment Station has had no recommendation for chemical control of this pest of pecans.
In 1966, control experiments were set up to determine if four applications of azinphosmethyl (Guthion®), EPN or phosphamidon would give satisfactory control of the hickory shuckworm under Texas conditions. The first application was applied August 16, and thereafter at 2-week intervals until each treatment received four applications.
A grove containing several named varieties of 13-to-15 year old pecan trees located near College Station, Texas, was selected on which to conduct this experiment. The experiment was arranged in a randomized block, using a split plot design and replicated twice. Eight trees, consisting of two trees each of four named varieties received each treatment, and eight corresponding trees were used as untreated checks. The four named varieties selected for this experiment were Moore, Desirable, Success and Stuart.
All treatments were applied by encircling each tree with a speed sprayer, applying from 17 to 20 gallons per tree.
About the time of harvest, shucks from 50 nuts were collected from each tree in the test, and data were obtained on the percentage of damaged shucks.
Results of the four late-season insecticidal applications are summarized in Table 2. Each of these insecticides was satisfactory in controlling the hickory shuckworm when applied as late-season sprays. All treatments had significantly less shuck damage than the check, and there was no significant difference in any of the treatments.
By combining the total damage of treatments and checks for each of the four different named varieties involved, it was shown that there was a difference in the amount of damage caused by the shuckworm to the different varieties involved in this experiment. The Moore and Desirable varieties had significantly less damage than the Stuart, but there was no significant difference between Moore, Desirable and Success, or the Success and Stuart, Table 3. This suggests that the shuckworm may exhibit an oviposition preference, survival difference, or some other factor influencing infestation rates which is present in certain of the named varieties.
Summary
The biology of the hickory shuckworm was studied under field conditions during 1966 and under laboratory and field conditions during the first portion of 1967. In the College Station area, five generations were completed during the 1966 growing season, followed by the overwintering generation. Data during 1967
Table 2. Comparison of three insecticides applied to control the hickory shuckworm on pecans.
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Number damaged
Lb. actual quarter shuck
Insecticide per 100 gal. sections per
50 nuts 1/
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Azinphosmethyl 0.5 7a
EPN 0.5 15a
Phosphamidon 0.5 34a
Check none 183b
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1/ Values followed by the same letters are not significantly different at the 5 percent level.
Table 3. Comparison of hickory shuckworm infestation rates in four improved varieties of pecans.
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Number damaged
Named varieties quarter shuck sections
per 50 nuts 1/
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Moore 26a
Desirable 47a
Success 63ab
Stuart 103b
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1/Values followed by the same letters are not significantly different at the 5 percent level.
substantially paralleled with that of the preceding year.
Three generations were reared under controlled laboratory conditions on an artificial diet, and data were obtained on the duration of the various stages of development, larval instars, etc.
Applications of azinphosmethyl, EPN and phosphamidon were applied to pecans as late-season sprays starting August 16, and then at 2-week intervals until the trees had received four applications. All insecticides applied showed a significant reduction in the number of damaged shucks at harvest, and the data showed no significant difference between the insecticides applied.
The combination of the damaged shucks from the treatments and the check for each named variety showed that Moore and Desirable varieties have significantly less shuckworm damage than the Stuart variety.
REFERENCES CITED
- Adkisson, P. L., E. S. Vanderzant, D. L. Bull, and W. E. Allison. 1960. A wheat germ medium for rearing the pink bollworm. J. Econ .Entomol. 53 (5): 759-61.
- Osburn, Max R., William C. Pierce, A. M. Phillips, John R. Cole, and George L. Barnes. 1963. Controlling insects and diseases of the pecan. USDA Agric. Handbook No. 240: 3.
