The Pecan Shuckworm
THE HICKORY OR PECAN shuckworm, Laspeyresia caryana (Fitch), is the most destructive insect attacking pecans. A native of the eastern United States, this pest ravages pecan orchards throughout the Pecan Belt, west to and including Texas.
This pest has four separate stages of development, of which the larva is the destructive stage. Shuckworms tunnel in and destroy the interior of the nuts until the nut shells harden. Nuts that become infested before the shells harden fall to the ground. Fifty percent or more of a pecan crop can be lost when there is a light set of nuts (9).After the nut shells harden, the shuckworms feed in the shucks surrounding the pecans, since the larvae are incapable of burrowing into the interior of the nut (1). Shucks infested early in the fall are likely to cause nuts to be poorly filled and later maturing than those not infested with shuckworms. Often injured shucks will adhere to nuts, which interferes with processing. Discolorations of the nut shells often result from shuckworm infestations and can lower the market value of the nuts (2).
In the northern United States the shuckworm has only one or two generations a year; in the southern states (at least the latitude of southern Georgia), four or five generations a year are very common (8). Determining the number of generations occurring each year in southern states is difficult because the life cycles overlap considerably. The shuckworm overwinters as a mature larva in pecan or hickory shucks (3). Most larvae pupate around the first of February, so a few moths will emerge as early as the middle of February (9). Duration of the pupal period can extend from 14 days to several months, although most adults have been found to emerge in late March and April (13). A small number of shuckworms may spend a second winter in old shucks (1). Small numbers of first generation larvae are found on pecans, since foliage and nuts of the pecan do not appear at the time most shuckworms emerge as adults. Hickories, which develop foliage 2 to 3 weeks earlier in the spring than pecans are a prime host of spring-brood shuckworms. Galls formed by certain species of Phyloxera on new foliage and nuts of hickory and pecan are also important alternative hosts (5). Eggs are deposited on foliage and nuts. Occasionally shuckworms may be found tunneling tender shoots of pecan or hickory. Damage to green nuts is greater on hickories than pecans because few pecans are infested with shuckworms until June.
Eggs laid in the summer months require 5 to 7 days to hatch (13). Larvae feed 19 to 32 days before pupating. Before pupation, the larvae tunnel to the outer covering of the husk to prepare an exit through which the adult can emerge. In the summer, the pupal period is 4 to 11 days. Starting in June and ending with the advent of cold weather, shuckworm populations increase rapidly with successive generations. Larvae mature in pecan and hickory shucks and overwinter in them until spring.
Use of biological organisms to control shuckworm populations has developed slowly since its beginning nearly 40 years ago. Adair (1) stated: "There are several parasites which attack the larvae of the shuckworm. Of these, Phanerotoma tibialis Hald., Calliephiltes grapholithae Cress., Microbracon cushmani Mues., Tachinophytes variabilis Cog., and a species of Perisierola are perhaps the most important. The egg parasite Trichogramma minutum Riley has been reared from eggs of this species during the past season. The subject of parasites also being investigated, but will require considerable investigation over a long period, and therefore, no statement relating to their utilization can be made at the present time."
Walker (13) stated that of the parasitic insects attacking shuckworms, Tachinidae is the number one enemy of the shuckworm, and Ichneumonidae ranks second. This author also mentioned Trichogramma minutum as having emerged from shuckworm eggs and that shuckworms are preyed upon by certain ground beetles and predacious mites. Walker listed a fungus as pathogenic to overwintering larvae. Release of Trichogramma minutum to control shuckworms was found to be inconclusive, if not totally ineffective (10). In their research on parasites attacking the pecan nut casebearer, Nickels et al (1) stated that Phanerotoma fasciata Provancher was the most important parasite attacking shuckworms in Texas. Perilampus fulvicornis Ashm. was found to be a hyper-parasite attacking several primary parasites.
Trevino (12) recently completed research on shuckworms in Mexico. In this study every parasite found attacking shuckworms belonged to the order Hymenoptera.
Procedure for Pecan Shucks
Pecan shucks were collected in December 1964 from five locations in Texas and one location in Louisiana. The Texas locations were pecan groves near Junction, College Station, Seguin, Mason and Eastland; shucks collected in Louisiana were near Mansfield. The shucks were held at room temperature until February 2, 1965, at which time they were placed in emergence cages outdoors. The emergence cages were cone-shaped structures, being 3x3 feet at the base and 2-1/2 inches in diameter at the apex. The cages were covered on all sides by wire screen and reinforced at four corners with wood stripping. A 1/2 pint jar was screwed vertically into the apex of the cage. A small wire screen funnel was inserted into the jar to prevent insects from leaving the jar, once they had entered.
Shucks collected from Louisiana were separated into six emergence cages. Vegetation was removed from the bottom of two of these cages to simulate clean cultured soil; two cages were placed on a sod culture; and the bottoms of the remaining two cages were covered with black polyethylene plastic, normally used as artificial mulch. Two separate lots of shucks were collected from Eastland, Texas; one lot of shucks collected from pecan trees that had received insecticide treatment, and the other lot from untreated trees. These two lots were placed in separate emergence cages. Shucks collected from the remaining four locations in Texas were each placed in a separate cage. All shucks collected in Texas were placed on clean cultured soil.
The jars on the emergence cages were collected and insects counted every 48 hours. Records were made on the number of shuckworm moths that emerged during these intervals as well as other insects emerging at the same time. Data on shuckworm moth emergence were made by accumulating the total number of moths emerging in 7-day periods. Temperature of the air was recorded as mean weekly high temperatures, as it was observed that moth emergence increased when relatively high daytime temperatures occurred. Temperature was recorded in degrees Fahrenheit.
Insects suspected of being parasitic on shuckworms were mounted, and then sent to the Entomology Research Division of the United States Department of
Agriculture for identification. Shuckworm moths, from each location where shucks had been collected, were also sent to the USDA taxonomists for positive species identification.
Fig. I. Weekly total of adult shuckworm emergence, Spring 1965.
Fig. 2. Weekly totals of adult shuckworm emergence, Spring 1965, Mansfield, Louisiana.
Shuckworm moth emergence data for the spring of 1965 is presented in Figure 1. This figure was made by recording, the number of moths emerging from shucks on clean cultured soil. Although it is not shown on either chart, one moth emerged as early as February 17, 1965, and some were still emerging when the test was discontinued July 1, 1965. Most of the moth emergence from shucks collected in Texas occurred in April. The peak of emergence was about the middle of April. The Louisiana moths had two peak emergence periods, compared to only one for Texas moths. The Texas moths emerged in greater numbers during the time period when Louisiana moths were emerging in relatively low numbers. Moth emergence was greatest immediately following the appearance of pecan foliage on March 29, 1965. Comparing the different locations in Texas, moths from Seguin, Texas had a peak emergence period about l week before the others.
Figure 2 represents data on emergence of moths from shucks placed on sod culture, clean culture, and plastic. Data on the emergence of moths from shucks placed on sod and clean cultured soil were practically identical. More moths emerged from the clean cultured cages, but there may have been more infested shucks in these two cages than in the sod cultured cages. The difference in the moth emergence among the three types of soil cultures was reflected only in the cages containing plastic. Moths emerging from shucks placed on plastic emerged, for the most part, in one large peak. The emergence data from cages having clean and sod cultures shows two peak emergence periods.
Several species of insects suspected of being parasitic upon the hickory shuckworm were found emerging with the adult shuckworms. A few of these insect species are known to attack hickory shuckworms, while others such as the example in Figure 3 may definitely be proved to be shuckworm parasites in future studies.
As a result of data compiled during the spring of 1965, several conclusions can be drawn about the emergence of the hickory shuckworm. Most of the moth emergence occurred after the pecan trees budded out and it continued through July. Shuckworms placed in cages on plastic had one peak emergence period around April 4-18. All shuckworms from Louisiana, except those placed on plastic, had two emergence peaks; whereas all shuckworms collected in Texas had one emergence peak. The two emergence peaks for shuckworms from Louisiana occurred around April 4-18 and May 9-23. The peak emergence period for shuckworms collected from five different locations in Texas occurred around April 18-25, which was between the two emergence peaks of shuckworms from Louisiana. From data compiled in a one-year study, it may be concluded that the emergence pattern of Louisiana shuckworms is different from that of Texas shuck- worms, and the emergence pattern of shuckworms on a plastic culture is different from shuckworms on sod and clean cultures.
Identification of Pecan Insects
All insects sent to the ARS branch of USDA in Beltsville, Maryland for the purpose of identification, have not returned at the time of this writing. Shuckworm moths, as well as at least three species of parasitic insects, have not yet returned from Maryland. One such species is shown in figure 3.Several species of parasitic insects that have been identified at this time are represented by five insect families. At last three species found in this test are known to parasitize shuckworms.
Fig. 3. Unidentified insect, suspected of being parasitic.
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*Graduate student who submitted this paper (printed in part) in partial fulfillment of requirement in Horticulture 685, Texas A&M University.