Selected Effects of Sublethal Dosages of Three Insecticides on the Bagworm

Methods and Materials

A Juniperus virginiana ‘Canaertii’ grove located on the Texas Tech University campus, Lubbock, Texas was selected as the test site. These trees had not been chemically treated the previous year. Bags remaining from the previous bagworm populations were removed by hand from all trees. Using a randomized design, four trees were selected for each treatment and the untreated check. Dosage determination trials were made using laboratory hatched populations. Female pupal cases filled with eggs were exposed to temperature regimes to facilitate early hatching (4). Larvae were reared in the laboratory for 30 to 60 days on branches of J. virginiana. Several different fractions of the recommended dosage for the tree test insecticides were given mortality ranking by placing larvae upon treated branches of J. virginiana. Larvae were confined on the branches within cages. Application rates were chosen which produced no more than a 50% mortality to assure adequate dosage to produce sublethal effect. The formulations and dosage rates selected were as follows: Diazinon AG 500^® at 1/8th the recommended dosage for bagworm control; Sevin^® 80% WP at 1Z16th of the recommended dosage; and toxaphene 8E at 1/16th of the recommended dosage.

Insecticide application was made at the test site using these rates with a John Bean® power sprayer set to deliver 400 pounds of pressure with a John Bean® gun. Eight hours after insecticide application, the test populations of larvae were placed on the treated trees. These larvae were approximately 35 days old based on initial field hatching dates and were gathered 24 hours before placement from a previously selected natural population occurring on J. virginiana which had received no chemical treatment during the previous year. Foil covered plastic cups fitted with air holes and food provided temporary housing for these larvae which were held in groups of ten.One hundred larvae were placed on each test tree.

Trees were monitored twice a week at 11:00 a.m. throughout the summer. Records were kept on the number of bags tied for pupation and the number of male pupal cases extruding from bags. The maximum daily temperature as recorded at the local weather bureau was used to determine if a positive correlation exists between high temperature and high frequency of male emergence numbers (6).

At the completion of the life cycle in the fall all bags were removed and the contents of each bag examined. Counts were made of male and female pupal cases, bags with larval remains which measured less than or more than one inch, and bags showing evidence of parasitism. Thirty-two female pupal cases from each treatment category were dissected for egg counts. Eggs were separated from pupal cases and surrounding scales following the method described by Morden (4). Eggs were rinsed in a 70 percent solution of ethyl alcohol and poured onto filter paper where dark and light colored eggs were counted using a dissecting microscope. Dark eggs were considered non-viable (8). Egg size was measured for 100 eggs chosen at random in each sample using a micrometer eye piece. As a second check of egg viability, a controlled temperature hatch using five bags per treatment was made. Although exact counts were not possible, observations were recorded.

Results and Discussion

Survival determinations were made on the basis of bag contents. An analysis of variance for treatment effect upon surviving adult means indicated no significiant difference (P 0.05). There was no significant effect upon survival between the sexes (P 0.05). An analysis of variance for treatment effect upon size of larvae at death provided no significant treatment difference (P 0.05). The number of parasitized larvae and pupae were judged too small for meaningful analysis.

In an examination of the mean numbers of bagworms to complete the pupal attachment, diazinon treatments differed significantly (P 0.01). Diazinon treated populations completed development through the pupation stage at higher numbers (Table 1).

Treatment effect on the number of eggs per female was not significant (P 0.05). Egg numbers ranged from 650 to 1550 and averaged 1100 eggs per bag. No significant differences were evident in an analysis of variance for vital and nonvital eggs (P 0.05). Observations indicated the egg viability to be essentially identical for the three treatments and the check.

All bagworms, regardless of treatment, followed the expected pupation and emergence patterns (3). There was no correlation between high temperature and high frequency of male emergence. These findings do not substantiate earlier emergence studies in Illinois (6). However, temperatures recorded in this study were approximatley 20° C above those in the Illinois study suggesting that a 42 ° to 50° C range may be too far above the minimum required temperature to show a correlation. The emergence period for the 1979 season in Lubbock, Texas encompassed 60 days and substantiates the longer emergence period exhibited by populations in the southern range (5).

Egg measurement suggested a significant treatment effect upon egg size (P 0.0003). Eggs averaged 1.2 mm in length as noted in an earlier study (7). The mean values for egg length (Table 2) indicated that eggs from toxaphene treatments were significantly larger than those from carbaryl treatments or untreated checks, but eggs from carbaryl treatments were significantly smaller than those of all other catagories. Egg size was measureably affected by sublethal dosages of two toxicants used. The effect on egg size noted in toxaphene and carbaryl treatments may have an indirect impact upon overwintering egg survival, subsequent larval survival and/or other effects which were not resolved in tests reported here.