Biological attributes and morphometric characters of egg parasitoid Trichogramma chilonis Ishii on Corcyra Ceohalonica Stainton

Introduction

Materials and Methods

Nucleus culture of T. chilonis and C. cephalonica

The factitious host, Corcyra cephalonica (Stainton), and the test insect, Trichogramma chilonis Ishii, were maintained at the Biocontrol Laboratory, College of Agriculture, Junagadh Agricultural University, Junagadh, Gujarat, under laboratory conditions of 27 ± 1 °C temperature and 70 ± 5% relative humidity as per the methodology put forwarded by Naganna and Shinde (2017 23 ) [8] with slight modification. The nucleus culture of C. cephalonica was maintained on a sorghumbased diet and the eggs collected from the culture were used for rearing T. chilonis. The nucleus culture of T. chilonis was maintained on C. cephalonica eggs using parasitized trichocards. Freshly collected C. cephalonica eggs were sterilized under ultraviolet radiation and approximately 1 cc of eggs was uniformly pasted on a 15 × 10 cm card using acacia gum. The cards were allowed to dry for 15 minutes and exposed to adult T. chilonis for parasitization. After 24 hours of exposure, the cards were transferred to clean glass bottles (22 × 11.5 cm) and maintained under laboratory conditions. Adult parasitoids were provided with 5 per cent honey solution as food.

Methodology for biological attributes

Biological attributes of Trichogramma chilonis on Corcyra cephalonica were studied following the methodology suggested by Mehendale (2009 18 ) [9] . Freshly laid, UV-sterilized C. cephalonica eggs (100 eggs) were uniformly pasted with diluted acacia gum on white paper strips measuring 5 × 2 cm and distributed using a pepper sprinkler. The egg strips were air-dried for 15 minutes and placed individually in plastic vials measuring 7.5 × 4 cm. A total of 30 egg strips were maintained for the study. Each vial was provided with five pairs of freshly emerged T. chilonis adults obtained from the laboratory culture. The parasitoids were supplied with 5 per cent honey solution as food, provided as small droplets on a plastic strip fixed inside the vial. The observations on biological attributes were recorded subsequently.

Total developmental period

The total developmental period was considered as the period between parasitization and adult emergence. After 24 hours of exposure to T. chilonis, each parasitized egg card was transferred to a clean container for further observations. The cards were observed daily until the parasitized C. cephalonica eggs turned black. The time taken from parasitization to blackening of the eggs was recorded in days. The blackened eggs were subsequently observed for adult emergence at 6:00 AM and 6:00 PM, at an interval of 12 hours, to determine the time of adult emergence accurately. The period from egg blackening to adult emergence was recorded in hours and later converted into days. The total developmental period was calculated as:

Total developmental period (days) = Time taken from parasitization to egg blackening (days) + Time taken from egg blackening to adult emergence (days)

Parasitisation potential

The parasitized eggs turned black after the fourth day of parasitisation, which was counted to calculate the parasitic potential. The actual number of parasitized eggs were obtained by subtracting the number of eggs left over from the total number of exit holes on C. cephalonica eggs and the total number of adults emerged from the egg strip.

Adult emergence

After complete emergence of T. chilonis adults, the total number of adults emerged from each parasitized egg card was counted irrespective of sex.

Adult longevity

The adults of T. chilonis emerging from parasitized C. cephalonica eggs were separated into males and females based on their morphological characters. A total of 30 males and 30 females were maintained individually in separate clean vials. Five per cent honey solution was provided as adult food. Individual parasitoids were observed at 6:00 AM and 6:00 PM until death. The period from adult emergence to adult death was considered as adult longevity, and male and female longevity was calculated separately.

Sex ratio

The adults emerging from the parasitized eggs were counted individually as male and female based on their morphological characters. The sex ratio was calculated from the number of male and female adults emerged from the laboratory culture and expressed as female ratio.

Total life cycle

The duration of the entire life cycle was considered as the period from the date of parasitization to the death of the emerged adult. The total life cycle of male and female T. chilonis was calculated separately using the following formula:

Total life cycle (days) = Total developmental period (days) + Adult longevity (days) ) [6] were considered for the present study. The measurements included body length and breadth, head capsule length and breadth, antennal length, forewing and hindwing length and breadth, hind tibia length and female ovipositor length. The morphometric observations were recorded as described below. The male and female body. The body length of T. chilonis was measured from the anterior portion of the thorax (prothorax) to the terminal end of the abdomen. The maximum abdominal breadth was considered as the body breadth of the respective sex.

Male and female head capsule (frontal view)

The length and breadth of the head capsule were measured after detaching it from the rest of the body. Head capsule length was measured from the vertex to the labium, whereas the maximum breadth near the compound eyes was recorded as the head capsule breadth.

Male and female antenna

The antenna was detached from the head before measurement. Since the antenna was curved, its total length was measured in two parts. First, the length of the scape plus pedicel was measured from the antennifer to the base of the flagellum in males and the club in females. Second, the length of the flagellum in males and the club in females was measured. The total antennal length was calculated by adding these two measurements.

Male and female wing

The forewing length was measured from the base of the wing to the point where the median anterior vein reached the wing margin. The hindwing length was measured from the base of the wing to its terminal end. The maximum width of both forewing and hindwing was recorded as wing breadth.

Male and female hind tibia

The length of the hind tibia was measured from the junction of the femur and tibia to the base of the tarsi.

Female ovipositor

The length of the ovipositor was measured from the basal portion of the first and second valvulae to the tip of the third valvula.

Results and Discussion

Biological attributes of T. chilonis

Trichogramma chilonis, a holometabolous hymenopteran wasp, passed through egg, larval, pupal and adult stages. As described for the genus, the immature stages are completed entirely inside the host egg: the parasitoid egg hatches in about 24 h, the larva passes through three instars, and the host egg darkens by the third instar, roughly 3-4 days after parasitisation, whereas the adult emerges through an exit hole and is free living (Irshad et al. 2025 12 ) [10] . A clear understanding of this biology is essential for the mass production and field use of the parasitoid. The biological attributes of T. chilonis reared on Corcyra cephalonica eggs at 27 ± 1 °C and 70 ± 5% relative humidity are therefore presented in Table 1 and discussed hereunder.

Egg-larval, pupal and total developmental period

The egg-larval period, taken as the time required for parasitised C. cephalonica eggs to turn black, ranged from 4.56 to 5.16 days with a mean of 4.85 ± 0.16 days, whereas the pupal period, from blackening of the host egg to adult emergence, ranged from 3.04 to 3.44 days with a mean of 3.23 ± 0.11 days. The total developmental period of T. chilonis on C. cephalonica was therefore 7.60-8.60 days, with a mean of 8.08 ± 0.27 days (Table 1). The present total developmental period is in close agreement with earlier reports on this parasitoid host combination. Perveen et al.

) [11] recorded 8 days for T. chilonis on C. cephalonica eggs at 28 °C, and Honnayya and Gawande (2018 11 ) [12] likewise noted a developmental period of 8 days on 24hrs, 48hrs and 72hrs old C. cephalonica eggs. Anoosha et al. (2025 2

) [13] reported development in about eight days, and Ajudia et al. (2023 1

) [5] documented successful development of the parasitoid on the same host. Perveen et al. (2012 26 ) [11] recorded 9.2, 8.0 and 7.2 days on C. cephalonica at 24, 28 and 32 °C, . The 8.08 days obtained at 27 ± 1 °C thus falls where expected, between the longer periods recorded at lower temperatures and the shorter periods recorded at higher temperatures. In the present study the host eggs turned black after a mean of 4.85 days, which is longer than the 3-4 days after parasitisation generally described for the genus (Khan et al. 2018 15 ; Irshad et al. 2025 12 ) [6,10] and the 4.00 days recorded for T. japonicum (Mohapatra and Shinde 2023 20 ) [7] . Correspondingly, the pupal period of 3.23 days was shorter than the 4.56 days reported by Mohapatra and Shinde (2023 20

) [7] . Because the totals are close, the differences lie mainly in how development is partitioned between the two phases. This de-pends on the criterion used to score blackening and on the interval at which egg strips are inspected, so the total developmental period is the more reliable basis for comparison among studies. The slight variation in developmental duration might be attributed to differences in Trichogramma species or strain, host species, host egg age, rearing temperature and the prevailing laboratory conditions.

Adult longevity

Female adults survived for 5.00-9.00 days with a mean longevity of 5.53 ± 1.25 days, whereas male longevity ranged from 3.00 to 5.00 days with a mean of 4.17 ± 0.83 days when adults were fed 5% honey solution. Females therefore lived about 1.4 days longer than males (Table 1). Female longevity in the present study is close to values reported earlier for T. chilonis: 5 and 6 days on Ephestia kuehniella and Plutella xylostella eggs, respectively (Miura and Kobayashi 1995 19 ) [14] , 5.6 days on S. cerealella at 31 °C (Nadeem et al. 2009 22 ) [15] , 4.9 days on C. cephalonica (Jalali et al. 2006 13 ) [16] , 4.8 days on S. cerealella at 24 °C (Sultan et al. 2013 34 ) [17] and 5.70 ± 1.59 days on C. cephalonica (Ajudia et al. 2023 1 ) [5] . Manickavasagam et al. (1994 17 ) [18] recorded 5 days for females and 4 days for males, in close agreement with the present female and male means. Male longevity (4.17 days) was lower than the 5.15 ± 1.04 days reported by Ajudia et al. (2023 1

) [5] but similar to the 4.6 days recorded for males by Rathi and Ram (2000 30 ) [19] . The longer survival of females than males agree with the pattern recorded for T. japonicum on C. cephalonica (4.90 ± 1.12 and 2.63 ± 1.10 days; Mohapatra and Shinde 2023 20

) [7] . Adult longevity is strongly influenced by nutrition. Mehendale (2009 18 ) [9] recorded 4.03 days for females and 3.00 days for males fed 10% honey solution. Temperature and host availability also affect lifespan (Irshad et al. 2025 12 ; Ghosh et al. 2017 7 ) [10,20] . The slight differences in longevity might therefore be due to adult food, the rearing medium of the host, ambient temperature, and the species or strain used. In the present study adults were provided only with 5% honey solution, so the values refer to honey-fed adults.

Total life cycle

The total life cycle of females ranged from 11.90 to 17.10 days with a mean of 13.61 days, whereas that of males ranged from 10.80 to 13.70 days with a mean of 12.25 days (Table 1). The mean female life cycle was 1.36 days longer than that of males, which corresponds to the longer adult longevity of females. The present life cycles are close to those reported for T. japonicum on C. cephalonica, 13.46 ± 1.21 days for females and 11.19 ± 1.11 days for males (Mohapatra and Shinde 2023 20

) [7] , the female value being almost identical. Because the life cycle is the sum of the developmental and adult periods, differences among studies may reflect the species tested, the factitious host, temperature, relative humidity and adult food.

Parasitisation

Parasitisation of the 100 C. cephalonica eggs offered per strip ranged from 82.00 to 93.00%, with a mean of 87.93 ± 3.03% (Table 1). The present mean is close to the 86.50 ± 5.09% reported by Ajudia et al.

) [5] for T. chilonis on C. cephalonica eggs and to the 85.33% recorded for T. pretiosum on the same host by Moirangthem et al. (2022 21 ) [21] . It is higher than the 60.67% recorded for T. japonicum on C. cephalonica eggs by Moirangthem et al. (2022 21 ) [21] . Anoosha et al. (2025 2

) [13] also reported high parasitisation by T. chilonis on C. cephalonica, whereas Funde et al. (2020 6 ) [4] reported lower parasitisation on the same host under their experimental conditions. The wide differences among studies indicate that parasitisation depends on the parasitoid species and on the host. Moirangthem et al. (2022 21 ) [21] found white and green trichocards most suitable for mass production of Trichogramma, and white card strips were used in the present study, which may have contributed to the high parasitisation recorded. Further variation among studies might reflect host egg age, parasitoid age and density, exposure period, host egg quality, temperature and relative humidity.

Adult emergence

Adult emergence ranged from 86.90 to 94.50% with a mean of 90.36 ± 1.95%, corresponding to 72-87 adults per 100 eggs exposed (mean 79.53 ± 4.39; Table 1). The relatively narrow range among the 30 replicates indicates consistent emergence under the rearing conditions used. Emergence was expressed relative to the number of parasitised eggs, as in Mohapatra and Shinde (2023 20 ) [7] , who related adults emerged to blackened eggs, which allows direct comparison. The present emergence is in line with several earlier reports: 89.4% on C. cephalonica and 90.13% on S. litura for T. chilonis (Dileep 2012 4 ) [22] , 88.30 ± 4.42% (Ajudia et al. 2023 1 ) [5] , 93.3% on C. cephalonica (Rathi and Ram 2000 30 ) [19] , 92.60% for T. japonicum at 26 °C on C. cephalonica (Ghosh et al. 2017 7 ) [20] and 86% for T. japonicum on C. cephalonica eggs (Pandi et al. 2021 25 ) [23] . Higher emergence was recorded for T. japonicum on C. cephalonica by Mohapatra and Shinde (2023 20 ; 96.67 ± 3.32%) [7] , whereas lower emergence was reported by Perveen et al. (2012 26 ; maximum 80.3% at 28 °C). Irshad et al. (2025 12 ) [10] , summarising Srivastava and Singh (2015 33 ) [24] , report that the highest fecundity and emergence of a temperature-tolerant strain of T. chilonis occurred at 27 °C within the range 25-31 °C, which is relevant to the 27 ± 1 °C maintained here. Most earlier findings thus support the present result; the slight variation might be due to the different factitious hosts, parasitoid species and the temperature prevailing during the investigations.

Sex ratio

The progeny sex ratio ranged from 1.71:1 to 2.70:1 (female: male) with a mean of 2.12:1 (Table 1). Expressed as male: female, as is common in the literature, this is 1:2.12 (range 1:1.71 to 1:2.70), indicating female-biased progeny. A similar female bias has been recorded earlier. Dileep (2012 4 ) [22] reported 1:2.10 for T. chilonis on S. litura and 1:1.53 on C. cephalonica eggs. Kumari et al. (2020 16 ) [25] recorded 1:2.50 on C. cephalonica at 25 °C, and Mohapatra and Shinde (2023 20 ) [7] recorded 1:2.82 (range 1:1.22 to 1:5.33) for T. japonicum. Mehendale (2009 18 ) [9] recorded 1:1.30 on C. cephalonica and 1:4.48 on Samia cynthia ricini. The present ratio therefore lies within the range of earlier reports. Some discrepancy among studies is expected because the sex ratio changes with the Trichogramma species, the host egg used (Mehendale 2009 18 ; Firoz 2015 5 ) [9,26] , host egg age (Dileep 2012 4 ) [22]

, the methodology and the prevailing environmental conditions, so the present value characterises this colony under these rearing conditions.

Morphometric characteristics of T. chilonis

Twelve morphometric characters of females and eleven of males of T. chilonis are presented in Table 2 and discussed hereunder. Females were larger than males in every absolute dimension, by 5.2% (head capsule length) to 8.2% (hind tibia length) when calculated from the means. The forewing L/W ratio was nearly identical in the two sexes (2.20 and 2.22), so the sexual difference lay mainly in overall size and not in wing shape. Larger females than males have been recorded in several Trichogramma species, and Mohapatra and Shinde (2023 20 ) [7] concluded that morphometric measurements help to differentiate the sexes of a species as well as the species themselves. Comparison with published values must, however, allow for differences in landmarks, specimen preparation and sample size. The morphology of Trichogramma also shows phenotypic plasticity over generations in laboratory culture (Pinto et al. 1989 28 ) [27] , which can contribute to differences among colonies.

Male and female body

Female body length was 466.8 ± 5.3 µm and male body length 441.5 ± 4.8 µm, while body breadth was 212.4 ± 3.2 µm in females and 199.6 ± 2.8 µm in males, so females were larger than males in both dimensions (Table 2). Larger females than males were likewise reported for T. chilonis by Khan et al. (2018 15 ; about 400 and 360 µm) and Yousuf et al. (2016 36 ; about 420 and 370 µm), for T. japonicum by Khan et al. (2018 15 ) [6] and Mohapatra and Shinde (2023 20 ; about 400 and 370 µm), for T. plasseyensis (396 and 378 µm; Hassan and Yousuf 2007 9

) [28] , for T. achaeae (543 and 478 µm; Khan and Yousuf 2017 14 ) [29] and for T. australicum (489 µm long and 177 µm wide in females against 454 µm and 165 µm in males; Dahlan and Gorth 1998 3 ). The ratio of female to male body length in the present study (1.06) lies within the range of 1.05-1.14 calculated from the means of the above studies. The absolute body lengths recorded here are greater than those given for T. chilonis by Khan et al. (2018 15 ) [6] and Yousuf et al. (2016 36 ) [30] and closer to those of T. achaeae and T. australicum, and they fall within the range of 342-640 µm reported for the genus (Hirai and Fursov 1998 10 ; Sumer et al. 2010 35 ) [31,32] . The slight differences in measurements might be due to species or strain, the nutritional value of the host, and the landmarks used to measure body length.

Male and female head capsule

Head capsule length was 138.7 ± 1.9 µm in females and 131.8 ± 1.6 µm in males, and breadth was 163.7 ± 2.4 and 154.2 ± 2.1 µm, respectively. The head capsule was broader than long in both sexes, as described for T. japonicum (Mohapatra and Shinde 2023 20 ) [7] , and the greater female head length agrees with the findings of Mohapatra and Shinde (2023 20 ; about 160 and 150 µm), Yousuf et al. (2016 36 ; 172 and 150 µm) [30] and Hassan and Yousuf (2007 9 ; 159 and 144 µm) [28] . Head breadth was almost the same in the two sexes in those studies (for example 207 and 205 µm; Yousuf et al. 2016 36 ) [30] , whereas females were about 6% broader-headed than males here. The head dimensions recorded here are smaller than the values given in the above studies, which might be due to species or strain, orientation of the head in frontal view, and the landmarks used.

Male and female antenna

Antennal length was 209.3 ± 2.7 µm in males and 134.4 ± 2.5 µm in females (Table 2). In Trichogramma the male antenna bears a long flagellum with long tapering hairs, whereas the female antenna ends in a short club, so the male antenna is the longer. The same pattern was recorded by Mohapatra and Shinde (2023 20 ) [7] , who reported total antennal lengths of about 210 µm in male and 130 µm in female T. japonicum. The present values are close to theirs, and the male antenna was about 1.55 times as long as the female antenna. Antennal length was thus the only character in which males exceeded females. For diagnostic purposes, separate measurement of the female club and male flagellum is recommended, together with the length and number of flagellar hairs, which separated T. chilonis (0.06-0.08 mm; 33-45 hairs) from T. japonicum (0.08-0.10 mm; 26-35 hairs) in males (Khan et al. 2018 15 ) [6] .

Male and female forewing and hindwing

Female forewing length and width were 413. ) [6] , 503 µm in both sexes (Yousuf et al. 2016 36 ) [30] and 500 and 490 µm in T. japonicum (Mohapatra and Shinde 2023 20

) [7] . The forewing L/W ratio recorded here (2.20-2.22) lies within the range given by Khan et al. (2018 15 ) [6] for T. chilonis (1.83-2.52; mean 2.01) and is close to their means for T. japonicum (2.27 in males and 2.34 in females), so wing proportion alone would not separate the two species in this colony. Khan et al. (2018 15 ) [6] found fore and hindwing width useful for separating T. chilonis from T. japonicum. The absolute wing values recorded here (forewing 393-414 µm long, hindwing 261-278 µm long) are smaller than those in the studies cited above. The differences are most probably due to the strain, the host and the way the wings were mounted and measured, and the present values should be taken as baseline values for the colony studied.

Male and female hind tibia

Hind tibia length was 135.4 ± 1.9 µm in females and 125.1 ± 1.7 µm in males, so that females had the longer tibia (Table 2). The same pattern was recorded in T. japonicum (about 160 and 140 µm; Mohapatra and Shinde 2023 20

) [7] and T. chilonis (175 and 170 µm; Yousuf et al. 2016 36 ) [30] , although Khan et

) [6] recorded equal tibia lengths (about 180 µm) in the two sexes of T. chilonis. The values obtained here are shorter than those for T. chilonis in the above studies and nearer to those of T. japonicum. Because hind tibia length is used as the reference length in several diagnostic ratios (Nagaraja and Nagarkatti 1973 24 ; Sorokina 1984 32 ) [33,34] , the difference might be due to the landmarks used, specimen mounting, host and strain, and it is advisable to follow a fixed landmark definition when ratios are compared among laboratories.

Female ovipositor

Ovipositor length of females was 158.2 ± 1.3 µm (Table 2). This lies within the range of about 150-200 µm (mean about 180 µm) given for T. chilonis by Khan et al. (2018 15 ) [6] and is somewhat shorter than the 178 µm recorded by Yousuf et al. (2016 36 ) [30] , whereas it is clearly shorter than the about 220-230 µm reported for T. japonicum (Mohapatra and Shinde 2023 20 ; Khan et al. 2018 15 ) [7,6] . The ratio of ovipositor length to hind tibia length is a recognised taxonomic character (Nagaraja and Nagarkatti 1973 24 ; Sorokina 1984 32 ) [33,34] . It was 0.90-1.16 (mean 1.01) in T. chilonis and 1.24-1.57 (mean 1.43) in T. japonicum in Khan et al. (2018 15 ) [6] , and the ovipositor was about 1.37 times the hind tibia in T. japonicum (Hirai and Fursov 1998 10 ; Mohapatra and Shinde 2023 20

) [31,7] . The ratio calculated from the present means is 1.17, which is just above the upper limit given for T. chilonis and well below the range given for T. japonicum, so the colony is closer to T. chilonis in this character. The slightly higher ratio is consistent with the shorter hind tibia recorded here (Section 3.2.5), because hind tibia length is the reference length of the ratio. It might also reflect the point from which the ovipositor was measured, the mounting of the specimen (whole mounts compared with dissected slide mounts; Platner et al. 1999 29 ) [35] , and strain or host effects, and the value should be treated as colony-specific baseline information and not as a diagnostic range for the species.

Conclusion

Trichogramma chilonis completed development on Corcyra cephalonica eggs in about 8 days (8.08 ± 0.27 days) at 27 ± 1 °C and 70 ± 5% RH, with high parasitisation (87.93 ± 3.03%) and adult emergence (90.36 ± 1.95%). Progeny were female-biased (2.12:1), and females lived longer than males (5.53 vs 4.17 days), which favours colony productivity. Females were larger than males in all absolute dimensions except antennal length, where males were longer (209.3 vs 135.4 µm). These results confirm C. cephalonica as a suitable factitious host and give baseline biological and morphometric data for this laboratory colony.

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