Lethal and sublethal effects of synthetic and bio-insecticides on Trichogramma brassicae parasitizing Tuta absoluta.

The tomato leaf miner (TLM), Tuta absoluta (Meyrick), is an invasive tomato pest found worldwide. Sustainable control strategies aimed at increasing biological control approaches and decreasing chemical inputs are required, due to the tendency to develop insecticide resistance. In this study, the lethal and sublethal effects of four chemical insecticides (abamectin, indoxacarb, chlorantraniliprole, and spinosad) and the sublethal effects of the entomopathogenic fungus Metarhizium anisopliae (Metschnikoff) on a widespread TLM egg parasitoid, Trichogramma brassicae Bezdenko, were estimated. Concentration mortality response bioassays enabled the estimation of lethal concentrations of the tested insecticides for the parasitoids, with chlorantraniliprole having the lowest LC50 and indoxacarb the highest. The LC25 and LC50 of the tested insecticides on the TLM were sprayed on eggs and then offered at three time intervals to the parasitoids. The fertility and other life table parameters of the individuals emerging from the treated eggs were estimated. All of the chemical insecticides, but not the fungus, had harmful effects on T. brassicae. The insecticide applications caused a 3.84-5.17 times reduction in the net reproductive rate (R0) compared with the control. No parameters were affected by spraying the fungus in the 0h treatment, but effects were recorded at 24 and/or 48h, except for the gross reproduction rate (GRR). The value of the intrinsic rate of increase (rm) also decreased to 0.528-0.617 after the insecticide treatments. The doubling time (DT) increased in all treatments compared to the control. Nevertheless, the generation time (T) was only very slightly affected. In addition, in the combination experiments, M. anisopliae showed a remarkable synergism with T. brassicae in controlling TLM eggs. These results indicate that low levels of lethal effects on key biological control agents should be considered in the choice of insecticides to be included in sustainable TLM control packages.

Comparative selectivity of nano and commercial formulations of pirimicarb on a target pest, Brevicoryne brassicae, and its predator Chrysoperla carnea.

Nanotechnology is a new field in the pesticide industry. Nanopesticides represent an emerging technological tool that offers a range of benefits including increased efficacy, durability, and reduction in the amounts of used active ingredients. However, due to the lack of studies on the toxicity and the sublethal effects on pests and natural enemies, the extent of action and fate of these nanopesticdes is still not fully understood limitting thus their wide use. In this study, we encapsulated the pirimicarb insecticide using nanostructured lipid carriers (NLC) and investigated the toxicity and sublethal effects (LC25) of the resulting nanocapsules against the cabbage aphid, Brevicoryne brassicae (Linnaeus) (Hemiptera: Aphididae) and its natural enemy the green lacewings Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae). Nanoencapsulation of pirimicarb enhanced 12.6-fold its toxicity to cabbage aphids compared to its commercial formulation. Furthermore, analysis of the age-stage, two-sex life table showed that negative effects on the B. brassicae aphid population growth were observed on F0 and F1 generations when aphids of parental (F0) generation were exposed to subelethal dose (LC25) of both formulations of pirimicarb. However, negative effects from sublethal exposure to the commercial and nanoformulated pirimicarb resulted in significant reduction on the net reproductive rate, intrinsic rate of natural increase, and finite rate of increase of the green lacewings C. carnea. Our findings indicate that the approaches and assumptions used to assess the risks of conventional insecticides may not apply for nanopesticides. Further research is still needed to better understand the environmental impact of these compounds.

Foraging egg parasitoids, Trissolcus vassilievi (Hymenoptera: Platygastridae), respond to host density and conspecific competitors in a patchy laboratory environment.

The egg parasitoid Trissolcus vassilievi (Mayr) is a key natural enemy of sunn pest, Eurygaster integriceps Puton (Hemiptera: Scutelleridae). This laboratory study examined how the aggregation of parasitoids varied in response to host density and numbers of foraging conspecifics. Five host densities (1, 2, 4, 8, and 15 clutches of E. integriceps eggs), were offered simultaneously to 1, 2, 4, 8, or 16 female wasps in a standardized arena (30 cm diameter) and patch residence times were recorded. Patch residence time was strongly correlated with parasitism, and increased as the number of host clutches increased up to eight, but declined when more clutches were offered. Wasps displayed low mobility and tended to remain in the patch initially encountered, even when it contained few egg masses. At higher wasp densities, patches were occupied more quickly, host exploitation began sooner, and per capita handling time was reduced. However, total patch residence times were similar across all densities. Thus, females responded to conspecifics with increased movement, which increased dispersal and resulted in a more homogeneous distribution of wasps among patches. Pseudo-interference, resulting from wasps remaining in some patches and neglecting others, had greater impact on final levels of host exploitation than did actual interference (patch abandonment following conspecific encounters). These phenomena resulted in decreased parasitoid searching rates and a 2-fold increase in host survival at higher wasp densities.

Relative Toxicity and Residual Activity of Nanocapsules and Commercial Formulations of Pirimicarb and Pymetrozine Against Myzus persicae (Hemiptera: Aphididae).

The green peach aphid, Myzus persicae (Sulzer), is one of the most common pest species that has the potential to transmit more than 100 plant viruses. Controlling this pest is difficult because it has become resistant to a wide range of insecticides. Nanoformulation has the capacity to reduce the pesticide load in agriculture and thus reduce the risks on human health and the environment. In this study, nanocapsules of pirimicarb and pymetrozine were prepared using nanostructured lipid carriers. The size, morphology, and encapsulation efficiency of nanocapsules were investigated using dynamic light scattering, scanning electron microscopy, and UV-VIS spectrophotometer. Zeta potential studies revealed stability of the nanocapsules of both insecticides. The encapsulation efficiencies were 85 and 81% for pirimicarb and pymetrozine, respectively. The nanocapsules were spherical with sizes of 35.38 and 35.12 nm for pirimicarb and pymetrozine, respectively. The LC50 values for the wettable powder (WP) and nanocapsule of pirimicarb after 48 h were 216.2 and 73.2 mg ai/l; for pymetrozine after 96 h, the values were 40.6 and 14.8 mg ai/l, respectively. Durations of residual activity for WP and nanocapsule formulations of pirimicarb were 7 and 15 d, respectively. The residual activity periods for WP and nanocapsule formulations of pymetrozine were 9 and 17 d, respectively. The results revealed that nanoencapsulation can improve performance allowing for reduced doses and increased duration of insecticidal activity for both of the insecticides tested.

Host Plant-Herbivore-Predator Interactions in Chrysoperla carnea (Neuroptera: Chrysopidae) and Myzus persicae (Homoptera: Aphididae) on Four Plant Species Under Laboratory Conditions.

The common green lacewing, Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae), is a well-known biocontrol agent. The current study examined host plant-herbivore-predator interactions with C. carnea and Myzus persicae on four host plants (peach, almond, pepper, and potato). The experiments were carried out at 25 ± 1°C and 65 ± 5% RH at a photoperiod of 16:8 (L:D) h). Duration of the preadult growth period, adult longevity, fecundity, and population growth parameters were analyzed based on the age-stage, two-sex life table theory. The shortest and longest preadult developmental times of the predator were observed on the peach and potato, respectively. The highest and lowest predation rate, oviposition period, and male and female longevity of predator were also observed on the peach and potato, respectively. The lowest intrinsic rate of increase (r) and finite rate of increase (λ) were observed on the potato (0.1087 and 1.11 d-1, respectively) and the highest on the peach (0.1460 and 1.15 d-1, respectively). The maximum and minimum mean generation times (T) were 41.84 and 35.59 d in the potato and peach, respectively. Overall, peach was found to be a more appropriate host than the other host plants for development and predation fitness of C. carnea. These findings reveal that information on tritrophic interactions and subsequent life table evaluation of natural enemies improves integrated pest management programs.

Life history parameters of the Sunn pest, Eurygaster integriceps, held at four constant temperatures.

The Sunn-pest, Eurygaster integriceps Puton (Heteroptera: Scutelleridae), is the most important insect pest of wheat and barley in Iran. A demographic study was carried out in order to determine the effect of temperature on life history parameters of the pest. Life tables were constructed at four constant temperatures: 22, 25, 27, and 30 ± 1° C using Mahdavi wheat kernels as food. Finite and intrinsic rates of population increase, gross and net reproductive rates, intrinsic rates of birth and death, generation time, doubling time, and lifetime female fecundity all varied significantly among temperatures. The intrinsic rate of natural increase, rm, increased linearly with temperature and was estimated to be 0.0126, 0.0381, 0.0541, and 0.0789 females/female/day, respectively, at the above-mentioned temperatures. Generation time ranged from 121 days at 22° C to 40 days at 30° C. Net replacement rate was significantly lower at 22° C than at other temperatures (4.6 vs. 22.2 to 25.8 females/female/generation). Lifetime female fecundity ranged from 123.1 at 22° C to 209.4 at 30° C. The thermal threshold for post-diapause pre-reproductive development was estimated to be 20 degrees C, and 66.8 degree-days were required for its completion.

Effect of host change on demographic fitness of the parasitoid, Trichogramma brassicae.

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.