Effects of Entomopathogenic nematode, Steinernema carpocapsae against Schizaphis graminum (Hemiptera: Aphididae)

The wheat aphid, Schizaphis graminum, is deemed one of the most significant grain pests, causing substantial damage to the country's wheat farms. Consequently, various biological, agronomic, and chemical methods are used to control wheat aphids, where chemical spraying is the most common method. The increasing use of chemical insecticides has resulted in several issues, including the emergence of insect resistance to pesticides, adverse effects on the environment, beneficial organisms, and humans, and the development of insecticide-resistant pests. In this regard, insect pathogens like nematodes may be preferable to chemical insecticides. This study aimed to evaluate the lethal and sublethal effects of entomopathogenic nematode Steinernema carpocapsae on the life history parameters and phenoloxidase enzyme activity of S. graminum by L-DOPA (L-dihydroxyphenylalanine) substrate. This enzyme activity is essential to the insect's innate immune response.

S. graminum was cultivated on a wheat host in a growth chamber maintained at 25 ± 2 °C, 60 ± 5% relative humidity (RH), and 16:8 light/day (L:D) hour photoperiods. Daily observations and recordings were used to track the development period and survival rate. In addition, for breeding the wheat aphid S. graminum, populations from the plant protection department's laboratory were utilized and transferred to wheat plants in a growth chamber.
Aphids infested the plants once they had reached their optimal size. Age-stage and two-sex life table analyses were used to analyze the raw data. TWOSEX-MSChart was used to evaluate the raw data based on the age stage and two-sex life table. In addition, blood cell phenol oxidase and hemolymph testing were performed. Samples of the enzyme were preincubated with phosphate buffer at 30 °C for 30 min, then 50 µl of 10 mM L dihydroxyphenylalanine (L-DOPA) substrate was added and incubated at 25 and 30 °C for 5 min. The phenol oxidase activity was measured using a 492 nm enzyme-linked immunosorbent assay (ELISA) reader device.

The bioassays indicated that the lethal concentration (LC50) of nematode treatment against the adults of S. graminum was 136 larvae/insect. The current study revealed that S. carpocapsae had a higher adult mortality rate than S. graminum. Results of the sublethal effects experiment showed that the pest's fecundity in S. carpocapsae treatment (19.07 offspring/female) was lower than the control (39.16 offspring/female). Furthermore, the studied nematode significantly reduced the population growth parameters of S. graminum. The intrinsic rate of increase for treated and control nematodes was 0.300 and 0.421, respectively, per day. Based on the results, the nematode increased the phenol oxidase enzyme's activity at 25 and 30 °C relative to the control.

Considering that chemical insecticides are accompanied by problems such as resistance and residual toxins in agricultural products, it is preferable to use entomopathogenic nematodes, which have a higher immunity to environment and natural enemies. The lethal concentration (LC50) study of the examined nematode showed that the nematode S. carpocapsae exhibited good disease-causing power on the adult female aphids of S. graminum.
The investigation into the effect of experimental nematode sublethal concentration on the growth parameters of the wheat aphid population revealed that in the nematode S. carpocapsae, the characteristics of the aphid population, particularly the intrinsic rate of population increase parameter value, decreased significantly. Sublethal concentrations of the studied nematode can slow or even stop the population growth of wheat aphids and significantly impact aphid fertility, survival, and life expectancy.
In this study, the phenol oxidase enzyme's activity significantly differed between treated and untreated aphids. It appears that the insect uses phenol oxidase enzymes to counteract the nematode's effect, and this enzyme plays a role in the wheat aphid's body when dealing with S. carpocapsae. The overall results revealed that S. carpocapsae exhibited high lethal and sublethal effects on S. graminum. After additional field studies, it can be recommended for use in biological control and an integrated pest management (IPM) program for this pest.