No abstract

Trichogramma brassicae (Bezdenko) is the most important species of Trichogramma parasitoids in Iran. The cotton bollworm, Helicoverpa armigera (Hubner) is a polyphagous insect pest that attacks many crops including cotton, maize, soybean, tomato, etc. The bollworm egg is a suitable target for many Trichogramma species. Factitious hosts such as eggs of the flour moth, Anagasta kuehniella (Zeller) and cereal moth, Sitotroga cerealella (Hubner) are used for mass rearing purposes. But a problem that arises sometimes in laboratory cultures is the development of a tendency toward laboratory hosts following a few generations rearing with them. This may tend to a low efficiency on target pest in field conditions. In this study the possibility of declining efficiency of the parasitoid on target pest by developing such a preference to alternative hosts in previous generations were investigated when the flour moth or cereal moth uses as laboratory host. Two generations of T. brassicae were reared on each of the mentioned hosts and then transferred to H. armigera eggs for two further generations. The intrinsic rate of natural increase as well as other life table parameters were used for monitoring fitness of the parasitoid at successive generations. Even generations were included to determine if previously rearing host affected parasitoid performance. Results revealed that host shift from cereal moth to bollworm caused a sudden fall in population growth parameters (both intrinsic rate of natural increase and net replacement rate). Further rearing on bollworm eggs led to a relapse in both parameters. No similar effect was observed in cultures initiated with the flour moth. As a whole, cereal moth was a more suitable host than flour moth.

Nitrogen is one of the most critical elements for plant and herbivore growth. This research aimed to investigate the bottom-up effect of varying nitrogen fertilization on the demography of the ectoparasitoid, Habrobracon hebetor (Say) on the tomato fruit worm, Helicoverpa armigera (Hübner) reared under different host-plant treatments. Six tomato plant cultivars (e.g., Kingston, Riogrand, Earlyurbana, Redston, Superstrain-B, and Primoearly and 4 Nfertilization levels (0, 2.1, 3.0, 3.9 g/pot nitrogen as urea 46%) were tested, using 5th instar larvae of H. armigera parasitised by H. hebetor under laboratory conditions. The mortality rate and developmental time of the immature stages of the parasitoid, as well as adult longevity and fecundity, were recorded. Data were analyzed based on agestage two-sex life-table theory. The results showed that the developmental duration of the parasitoids was affected by the increase in the levels of nitrogen. The highest value was observed at 12.83 ± 0.11 days on Earlyurbana with the lowest nitrogen level. The lowest intrinsic rates of increase (r) and finite rates of increase (λ) were 0.1058 ± 0.0147 and 1.1013 ± 0.0157 day − 1 , respectively, regarding the lowest (zero) level of added fertilizer. However, the highest rates were 0.1515 ± 0.018 and 1.1608 ± 0.013 day − 1 , respectively. The fecundity and longevity of adult wasps were positively correlated with nitrogen levels. Accordingly, differences in host-plant quality can indirectly affect the performance of parasitoids via the herbivore.

The current survey was carried out to evaluate the effect of different nitrogen levels (0, 2.1, 3.0, 3.9 g • pot -1 nitrogen as urea 46%) on tomato fruit worm Helicoverpa armigera on six common tomato cultivars (e.g., Kingston, Riogrand, Earlyurbana, Redston, Superstrain-B and Primoearly) under laboratory conditions [25 ± 1°C, 60 ± 5% RH, 16 : 8 (L : D) h]. The mortality, developmental period of immature stages as well as the longevity and fecundity of adult stages were recorded. Data were analyzed based on the age-stage, two-sex lifetable theory. The longest (24.21 ± 0.59 days) larval developmental period was recorded in Earlyurbana variety with zero nitrogen level and the shortest (15.44 ± 0.36 days) in Superstrain-B variety with the highest nitrogen level. Consequently, the net reproductive rate (R 0 ) ranged from 35.7 ± 7.06 to 62.16 ± 18.9 offspring/female/individual in Redston variety with zero nitrogen level and in Superstrain-B variety with the highest nitrogen level, respectively. The lowest and highest values of the intrinsic rate (r) and finite rate of increase (l) were estimated for Redston variety with zero level of nitrogen (0.0712 ± 0.0065 and 1.0732 ± 0.0069 day -1 ) and Superstrain-B variety with the highest nitrogen fertilizer (0.1507 ± 0.0057 and 1.1629 ± 0.0066 day -1 ), respectively. The results demonstrated that nitrogen fertilizer influenced nearly all the life parameters of the pest which depended on the cultivars. Finally, it could be concluded that Kingston and Superstrain-B were suitable and Earlyurbana and Redston were unsuitable host plant cultivars for H. armigera.

No abstract