Numerous life history studies of the potato tuberworm, Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae), a serious pest that affects potato production worldwide, have been done (Langford & Cory 1932; Mukherjee 1949; Bartolini 1951; Stanev & Kaitazov 1962; Al-Ali et al. 1975; Foot 1979, Chauhan & Verma 1991; Rondon 2009). Mukherjee (1949), Bartolini (1951), and Chauhan & Verma (1991) studied dimorphism of this species. Sexual dimorphism does not become evident until yellowish testes become visible in the fourth instar; gradual change in eye pigmentation shows different pupal stages (yellow, early red, middle red, late red, and black eye pupa) (Chauhan & Verma 1991). Mukherjee (1949) found that male pupae have a round “pit-like genitalia aperture” in the middle of the 9th segment, while females have a single aperture on the 8th segment. No one has provided a practical description of pupal differentiation. Our objectives were (1) to sex male and female P. operculella pupae and adults by using external morphological characteristics; and (2) to determine the accuracy of sex ratios by comparing 2 different sexing methods.

Rondon et al. (2009) protocol was followed to rear P. operculella pupae and adults. Forty-eighth-old pupae in silken cocoon were collected from the colony, and pupae were freed from the cocoon by immersing pupae in 1% aqueous bleach (Dogramaci et al. 2008) for about 2 min, after which clean pupae were harvested. We used the “width” and “scar” method to sex the pupae. The last abdominal segment is round in males and pointed in females (“width” method), while there is a suture on the 8th or 9th abdominal segment (“scar” method). Twenty pupae were randomly separated into 4 groups and replicated 5 times (n = 400 pupae). Each experiment was conducted by 3 samplers to test accuracy in sampling. Fortyeight-h-old adults were sexed based on dark-spot patterns or the “wing” method (2–3 dots on males; “X” on females) and end of the abdomen shape or the “genitalia” method (thick set of hairs on males). Fifteen adults were randomly separated into 4 blocks replicated 5 times (n = 300 adults). All analyses were conducted with the statistical software program SAS version 9.1 (SAS Institute 1997). In both pupal and adult studies, a paired t test was used to determine if the sex ratios obtained from the 2 sexing methods were significantly different. A generalized estimating equation (GEE) (SAS Software) method was used to compare the number of unclassified cases between 2 methods and to assess whether there was a sampler effect. The GEE was used to account for possible correlation arising from the fact that 3 samplers were checking the same samples. It was performed by PROC GENMOD in SAS.

Male pupae measured 6.4 ± 1.2 mm in length, while female pupae measured 6.6 ± 1.1 mm in length. Females have a suture located approximately 0.59 ± 0.10 mm from the end of the abdomen and males have the suture at 0.38 ± 0.08 mm. The number of females and males determined from both characteristics are summarized in Table 1 for each of 3 different samplers. The “width” method has fewer unclassified cases (P < 0.0001) than the “scar” method with the averaged percentages of unclassified for the “width” and “scar” methods being 0.88% and 20.22%, respectively. A sex ratio of 0.90 was established by averaging “width” sex ratios from 3 samplers. The sexing results based on 2 methods were not different (P = 0.2708) with averaged sex ratio from the “width” and “scar” methods being 0.90 ± 0.01 and 0.83 ± 0.04, respectively. Table 2 shows that only about 83% of the 182 unclassified cases by the “scar” method were classified as female by the “width” method, which explains the lower female to male ratio by the “scar” method.

Adult sexes differ from each other based on dots present in the forewings and tips of the abdomen including genitalia. Females present a characteristic patch of dots in a “cross shape” on the wings, while males present 3 sets of clearly defined dots. The adult male's terminal abdominal segment is broader than that of adult females. The number of females and males determined from both characteristics are summarized in Table 1 for each of 3 different samplers. Similar to the pupal study, the “genitalia” method was found to have fewer unclassified cases (P < 0.0001) than the “wing” method, with the average of unclassified for the “genitalia” and “wing” methods being 0.89% and 12.56%, respectively. A sex ratio of 0.95 was obtained by averaging “genitalia” sex ratios from 3 samplers. The sexing results based on the 2 methods were different (P = 0.0442) with average sex ratios from the “genitalia” and “wing” methods being 0.95 ± 0.01 and 0.82 ± 0.03, respectively. Table 2 shows that about 73% of the 113 unclassified cases by the “wing” method were classified as female by the “genitalia” method.

TABLE 1.

RATIOS OF FEMALE/MALE P. OPERCULELLA PUPAE AND ADULTS BASED ON MORPHOLOGICAL CHARACTERISTICS FOR 3 SAMPLERS (N = 400 PUPAE, 300 ADULTS).

TABLE 2.

AGGREGATED RATIOS OF PUPAE AND ADULTS MALES AND FEMALES FROM 3 SAMPLERS FOR P. OPERCULELLA TUBERWORM PUPAE.

We thank Chiho Kimoto, Sara Carlson, and Sandra DeBanno for assistance. We thank Mary K. Corp and Arnold P. Appleby (Oregon State University), and Oscar Liburd (University of Florida) for reviews of the manuscript. This research was supported by the Agricultural Research Foundation.

SUMMARY

Sexual dimorphism between P. operculella females and males is evident in the pupal and adult stage. Male pupae are slightly smaller than female pupae and have a suture located further away from the end of the abdomen. The “scar”/ “width” (pupae) and “wings”/”genitalia” (adults) methods are not entirely consistent. The “width” and the “wings” methods give a lower ratio than the “scar” and “genitalia” method, therefore overestimating the male/female ratio.