Tomato, Lycopersicon esculentum (Miller) (Solanaceae), is one of the most economically important crops in Ethiopia. Various insect pests are reported to attack tomatoes worldwide (Lange and Bronson, 1981). Some of the pest species are known to be of great economic importance; among them are the tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) and tomato fruit borer, Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae), which are very serious insect pests of tomato plants.
Tomato leaf miner, T. absoluta (Meyrick) is a native devastating pest of South America, particularly to tomato, L. esculentum (Miller) (Desneux et al., 2010; Gontijo et al., 2013). It may be described as an intercontinental pest. Although T. absoluta is an endemic neotropical pest, it has acquired a wider geographical distribution after its unintended introduction in other tomato production regions since its first detection in Europe in late 2006 (Urbaneja et al., 2007)
Over the past decade, a substantial amount of research in South America has addressed a wide range of topics related to T. absoluta biology, ecology, impacts and management. Since its unintentional introduction from Mediterranean region through Sudan to Ethiopia, this invasive pest has devastated indescribable thousands hectares of tomato crop. It is considered to be one of the most important Lepidopterous pests associated with tomato, in Ethiopia (Gashawbeza and Abiy, 2013). Larvae of this insect are known to feed on the leaf mesophyll tissue expanding miners, and fruits of the crop with subsequent reduction of the yield. It has a high reproductive rate, where the female lays 260 eggs during its life (EPPO, 2005) on tomato plants. Few authors have studied the life tables of T. absoluta on tomato plants (Miranda et al., 1998; Pereyra and Sànchez, 2006; Aksu and Çıkman, 2014; Erdogan and Babaroglu, 2014). It has become an increasingly important pest in Ethiopia when information on its biology was discovered, even in its native range which was scarce. From the perusal of the literature, this work is the first in Ethiopia on the Ethiopian strains of T. absoluta reared on tomato host plant. Therefore, the objective of the present work is to study the biology and oviposition of T. absoluta on the tomato plants under laboratory and glasshouse conditions.
The biology of T. absoluta on tomato was studied under laboratory and glass house temperature; 20.5±2°C and 55±5% R.H. in the laboratory and 32.0±2°C and 40±5% R.H. in the glasshouse. T. absoluta larvae were collected from the fields and brought to the laboratory and glasshouse. The tomato leaf miner larvae present on these collected tomato leaves were wrapped with wet cotton kept in plastic box (20 x 15 cm2) in the laboratory and glasshouse. After the emergence of the adults, cages were prepared under glasshouse and laboratory. Newly emerged adults were collected from the ovipositing female of laboratory culture and placed on the tomato plant in the cages.
Adult T. absoluta used in this study was obtained from the culture maintained in the laboratory with temperature and relative humidity, under natural light. Ten pairs of T. absoluta from this stock culture were sexed and released into a rectangular box cage (1× 0.6 m). T. absoluta was maintained in the glasshouse. A mixture of sugar, yeast and water was placed in a Petri dish as a food supplement in the cage for adult T. absoluta. Everyday, temperature, relative humidity, number of eggs, larva, pupa and oviposition period per females, sex ratio and longevity of female and male were recorded both under laboratory and glasshouse conditions.
Life table construction
The number of alive and mortality of T. absoluta in all the stages was recorded daily. The following assumptions were used in the construction of the life-table of T. absoluta.
qx= [dx / lx] x 100
Where x = Age of the insect in days; lx = number of survival at the beginning of each interval; dx = number dying at age interval; qx = mortality rate at age interval x calculated using the following formula
ex = Tx/ lx
ex = Expectation of mean life remaining for individuals of age x. Life expectation was calculated using the equation:
Lx = lx +1 (x + 1)/ 2
To obtain ex, two other parameters Lx and Tx were also computed as follows:
Lx = The number of individuals alive between age x and x + 1 calculated by the equation.
Tx = lx + (lx + 1) + (lx + 2)………………..+ lw.
Where lw = The last age interval; Tx = the total number of individual of x age units beyond the age x obtained by the equation.
Developmental stages of specific life table
Data on stage specific survival and mortality of eggs, larvae, pupae and adults of T. absoluta were recorded from the age specific life-table. x = Stage of T. absoluta; lx= number of survival at the beginning of the stage x; dx = Mortality during the stage indicated in the column x.
The data calculated through the above assumptions for computing various life parameters such as apparent mortality (Am), survival fraction (Sf), mortality survivor ratio (MSR), indispensable mortality (IM) and K-values were calculated according to Arshad and Parvez (2010).
All the necessary data were recorded and analyzed with calculated means of days. Data collected were subjected to analysis of variance (ANOVA) to determine the significance differences among the treatments using SAS Programme version 9.1 (SAS, 2009). Life table analyses were calculated according to Jackknife’s method (Sokal and Rohlf, 1995).
Eggs laying positions of T. absoluta on tomato plants
Results showed that oviposition preferences were significantly (P < 0.05) different from the others. All adult female T. absoluta laid their eggs on the upper side of the tomato leaves (60.56%) followed by the lower side (35.21%). The lowest parts of the plant where eggs were laid are as follows: stem (0.85%), flowers (1.41%) and fruit (1.97%) (Figure 1).
Developmental stage of T. absoluta
Significant (P < 0.05) differences were observed between laboratory and glasshouse studies on the life cycle of T. absoluta in both seasons. The female T. absoluta laid from 177.5 eggs/female at 32±2°C (40±5% R.H.) to 233.75 eggs/female at 20.5±2°C (55±5% R.H.) during her life span in 2015. Similarly, in 2016, the female T. absoluta laid 211.25 and 168.25 eggs at 20.5±2°C (55±5% R.H.) and 32±2°C (40±5% R.H.), respectively. Most of the eggs were embedded on the upper and lower leaf. Similar to our results, Uchoa-Fernandes et al. (1995) stated that the oviposition period was 7 days after first mating with a maximum life time fecundity of 260 eggs per female.
At an average of two consecutive seasons after 13 to 13.5 days, the eggs were hatched for the first instar larva at 20.5±2°C (55±5% R.H.), while at 32±2°C (40±5% R.H.) it took 10 to 10.5 days. The finding disagrees with the previous work of Erdogan and Babaroglu (2014), EPPO (2005) and Torrest et al. (2001) that the period of egg hatching was between 4-5 days.
A total of larval instars were observed in this study; it took 12.5 days at 20.5±2°C (55±5% R.H.) in the laboratory and 11 to 11.51 days at 32±2°C (40±5% R.H.), respectively. The results were confirmed by the work of Erdogan and Babaroglu (2014). They found that total period of larvae instar was 10.97 days; Pereyra and Sanches (2006) reported that the period of larvae instar of T. absoluta was 12.14 days at 25±1°C. It was also determined that the period of larvae instar of T. absoluta was 13-15 days (EPPO, 2005). Torrest et al. (2001) stated that the period of larvae instar of T. absoluta was 12 and 16 days at 27°C.
The developmental time of pupa ranged from 8.2 to 9.8 days at temperature of 20.5±2°C (55±5% R.H.) and 6.5 to 8.6 days at 32±2°C (40±5% R.H.) (Figure 2). Gadir et al. (2016) stated that the pupal development time of the tomato leafminer varied from 7.07 to 8.62 days on two tomato cultivars. It was found that the period of pupae instar of T. absoluta was 9.53 days (Erdogan and Babaroglu, 2014). Torres et al. (2001) also found the pupal developmental time of T. absoluta as 7 to 9 days. These studies are similar to the current findings.
The total mean of developmental period for T. absoluta from egg to adult was 30.6 to 31.2 days at 20.5±2°C (55±5% R.H.); while at 32±2°C (40±5% R.H.), it ranged from 26.4 to 27.8 days under glasshouse conditions (Table 1 and 2). The results agreed with the findings of Erdogan and Babaroglu (2014) showing that means of developmental period from egg to adult was 30.18 days. EPPO (2005) reported that under optimal conditions, T. absoluta developed in about 30 days. Barrientos et al. (1998) also found that average development time of T absoluta was 23.8 days at 27.1°C. Cuthbertson (2011) reported that the development from egg to adult took 35 days at 25°C. He also mentioned in the same year in England under greenhouse conditions, the mean total development time of T. absoluta was 39.8 days at 19.7 °C. Erdoghan and Babaroglu (2014) showed that the mean total development time of T. absoluta was 30.18 days on tomato at 25–26°C. T. absoluta developed slightly faster in this study compared to the results obtained by other researchers. This might in part be attributed to the tomato cultivar differences and possible differences in the population of T. absoluta in these results. According to Du et al. (2004), the developmental period of the herbivore insects is strongly affected by the nutritional qualities of the host plant, which in turn influences its population growth. On the other hand, the chemical components of host plants can also affect the survival, growth and reproduction of herbivore insects (Wilson and Huffaker, 1976; Bernays and Chapman, 1994; Adebayo and Omoloyo, 2007).
Age specific developmental stages
Mortality percentage of eggs was recorded during life cycle: 1.40% at 20.5±2oC (55±5% R.H.) and 4.93% at 32±2°C (40±5% R.H.). But Cuthbertson (2011) stated that in the development stage, the survival of the egg stage was 100%; no mortality rate was recorded. The highest larval mortalities (26.49%) were recorded at 30.5±2°C (40±5% R.H.), while the lowest mortalities (15.99%) were at 20.5±2°C (55±5% R.H.) under glasshouse and laboratory, respectively. The pupal mortality rates during the study periods were recorded as 10.23 and 32.42% under laboratory and glasshouse conditions, respectively. Erdogan and Babaroglu (2014) found that the survival rate of pupa was 63.10% that is, 36.9% mortality. It was very high compared to that of this result (Table 3). From these studies, it is understood that at high temperature, the mortality percentages of all stages increased during developmental periods of T. absoluta under both conditions.
Significant (P < 0.05) differences between treatments of survival percent of eggs were recorded during their life cycle from eggs to larval stage. 143 eggs were hatched. Among these, 137 eggs emerged to first instar larvae (95.8%) and 6 eggs died due to unknown reasons. There were no significant (P > 0.05) differences between treatments, regarding larval survival in both conditions. Among 137 larvae hatched, 124 emerged to pupae (81.01%) and also, from the hatched pupae, only 109 emerged to adults (89.77) at 20.5±2°C (55±5% R.H.). On the other hand, at the same time, 142 eggs were collected and hatched under glasshouse, at 32±2°C (40±5% R.H.). Among the hatched eggs, 119 emerged to larvae (83.80%). From the 119 hatched larvae, 92 emerged to pupae (81.51%). Finally, among the hatched pupae, 72 adults of T. absoluta emerged to adults (78.26%) during 2015-2016 (Table 3).
Apparent mortality (AM)
Maximum apparent mortality was observed in fourth instar larvae (11.68%), while minimum mortality was recorded in second instar larvae (0.72) at 20.5±2°C (55±5% R.H.). Also, maximum apparent mortality was observed in last pupal stage (21.74%) while minimum mortality was recorded in third instar larvae (1.65%) at 32±2°C (40±5% R.H.), from 2015-2016. When a comparison was made between larval instars, the highest mortality (11.68%) was observed at fourth instar, whereas, minimum mortality was recorded at second instar (0.72) at 20.5±2°C. Similarly, mortality at pre pupa and pupal stages remained minimum (7.03 and 3.23%) at 20.5±2°C, respectively. In a similar way, at 32±2°C (40±5% R.H.), maximum mortality was recorded as 10.68% while minimum mortality was 21.74%, respectively (Table 4 and 5). In the present findings, the early pre -pupae were higher than the later pupa, and hence, showed higher mortality at pre -
pupae stage, which was observed at 20.5±2°C (55±5% R.H.). However, in glasshouse experiment at 32±2°C (40±5% R.H.), the highest mortality of T. absoluta was observed at last pupal stage.
Survival fraction (Sx)
In Tables 4 and 5, the maximum survival fraction was 0.99, from egg stage to third instar larvae at 20.5±2°Cand the minimum survival fraction was 0.88 at 30±2°C. Among larval instars, the Sx was very high at 0.99, but it was very minimum (0.88) during pupal stage at 20.5±2°C (55±5% R.H.). On the other hand, at 32±2°C (40±5% R.H.), maximum Sx (0.98) was obtained, but it was very minimum at pupal stage (0.78). Similarly, in another experiment, much lower survival of T. absoluta was obtained from 20 to 23°C (Miranda et al., 1998).
Mortality survivor ratio (MSR)
Mortality survival ratio (MSR) at egg stage was minimum (0.014) at 20.5±2°C (55±5% R.H.) and maximum (0.05) at 32±2°C (40±5% R.H.). Mortality survival ratio of larval instar at 20.5±2°C (55±5%) indicated that 0.007 was found at second and third instar larvae and 0.117 was found at fourth instar larvae. While at 32±2°C (40±5% R.H.), minimum larval instar was 0.02 at third instar larvae and 0.13 at fourth instar larvae. Between the pre- pupa and pupa, maximum mortality survival ratio (0.23) was observed at pre- pupa and minimum mortality survival ratio (0.11) was observed at 20.5±2°C (55±5%).
On the other hand, minimum (0.032) ratio was obtained at last pupal stage and 0.07 at pre- pupal stage at 32±2°C (40±5% R.H.). In the current study, T. absoluta showed high natural mortality during its life cycle at larval stage and pupa knownto be the most critical. Therefore, the findings on the mortality performance of T. absoluta corroborate with the study of Miranda et al. (1998).
Indispensable mortality (IM)
Indispensable mortality at egg stage was observed to be maximum (12.75) at fourth larval instar and the minimum results were recorded (0.76) at second and third larval instars at 20.5±2°C (55±5% R.H.). While comparing indispensable mortality between pupal stages, the maximum result (7.63) and minimum (3.49) at maximum (7.83) was observed to be similar in temperature and relative humidity. At 32±2°C (40±5% R.H.), the lowest value (1.44) was recorded at third instar and the maximum value was observed (16.56) at pupal stage (Tables 4 and 5).
The k-value was found to be minimum (0.0061) and maximum (0.0487) at 20.5±2 (55±5% R.H.) and 32±2°C (40±5% R.H.) during egg stage, respectively. While comparing larval instars, there was maximum ‘k’ (0.0295) at fourth instar and lowest ‘k’ (0.0030) at third instar larvae at 20.5±2°C (55±5% R.H.). In the case of pupal stage in both conditions, the highest k-value was recorded at last pupal stage (Tables 4 and 5). The total generation mortality k-value was recorded as maximum (0.3980) at 32±2°C (40±5% R.H.) and minimum (0.1178) at 20.5±2°C (55±5% R.H.). Similar findings were obtained for Mediterranean fruit fly (Carey, 1982).
Adult longevity of T. absoluta
Results from the study revealed that there was a significant (P < 0.05) difference in longevity of T. absoluta in all the treatments. There was no significant (P > 0.05) difference in longevity when female T. absoluta lived (17.2) at mean temperature and relative humidity of 20.5±2°C (55±5% R.H.), while the male T. absoluta lived for 8.9 days with an average of two seasons. On the other hand, at temperature and relative humidity of 32±2°C (40±5% R.H.) in the glasshouse, male and female T. absoluta lived for 6.5 and 14.8 days, respectively. In this temperature range, the life cycle of male and female T. absoluta is shorter compared to the other temperatures (Tables 1 and 2). The findings are close to the previous studies of Estay (2000), that observe that adult T. absoluta lifespan ranged between 10 and 15 days for females and 6 -7 days for males. It was found that adult longevity for male and female individuals was 15.8 and 18.16 days, respectively (Erdogan and Babaroglu, 2014).
The results are similar to those reported by Torrest et al. (2001) that the developmental period of larval instar of T. absoluta was 12–16 days at 27°C. The findings at 20.5±2°C (55±5% R.H.) showed that in both seasons, 13 to 13.5 days at 32±2°C (40±5% R.H.) were shorter than an average of 11 to 11.5 days. This study is in agreement with the findings of Pereyra and Sanches (2006) that the period of larvae instar of T. absoluta was 12.14 days at 25±1°C. It is resolute that the developmental period of larvae of T. absoluta was 13 to 15 days (EPPO, 2005). The two seasons results of pupal developmental stages revealed that it took 8.2 to 9 days at 20.5±2°C (55±5% R.H.) and 6.5 to 8.6 days at 32±2°C (40±5% R.H.). EPPO (2005) found that the period of pupae of T. absoluta was an average of 9.53 days. The means of development period from egg to adult was 30.18 days. But the findings showed that at 20.5±2°C (55±5% R.H.), it was 30.6 to 31.2 days and at 32±2°C (40±5% R.H.), it was 26.4 - 27.8 days. EPPO (2005) also reported that under optimal conditions, T. absoluta developed in about 30 days. We also agreed with Barrientos et al. (1998) that average development time of T. absoluta was 23.8 days at 27.1°C. Similar results were found by Fernandez and Montagne (1990). They reported that females lived longer than males, allowing them to be sexually mature when the males emerge.
One hundred eggs were collected and hatched to determine male to female sex ratio under both laboratory and glasshouse conditions. Among the hatched eggs, 48 males and 32 females emerged at 20.5±2°C (55±5% R.H.) in the laboratory, while the same number of eggs was hatched at 32±2°C (40±5% R.H.) in the glasshouse. Among these, 40 male and 36 female adults emerged from pupae. The total survival of T. absoluta adults in the laboratory was 80%; 3:2 male to female sex ratio. Although in the glasshouse the total survival of adults was less than those in the laboratory, it was shown that 76% adults survived in 40 male and 36 female (10:9) ratio recorded. In this finding, it was stated that usually, males were more than female individuals. The result confirmed the work of Cuthbertson (2011), which showed that males were more than females.