Effects of injudicious use of spirotetramat on Encarsia formosa's ability to control Bemisia tabaci.

The excessive and frequent use of insecticides has led to serious problems with insecticide residues, impacting nontarget organisms such as the parasitoid Encarsia formosa. This study examined the growth, development, and enzyme activity of E. formosa exposed to spirotetramat at LC10, LC30, and LC50. The regression equation for the toxicity of spirotetramat toward E. formosa was Y = 5.25X-11.07. After exposure to spirotetramat, the survival rates of E. formosa sharply decreased, which occurred earlier than those in the control batch. Although the maximum daily parasitism quantity of E. formosa increased and the average parasitism number, enumerated from the 1st to the 5th day, was 53.97 after being exposed to spirotetramat at LC10, the life span of its F1 generation adults was only 8.47 days, which was significantly shorter than that in the control batch. After being exposed to spirotetramat at LC50, the average parasitism number of E. formosa was 63.30, and the developmental time of its F1 generation, enumerated from the 1st to the 5th day after exposure to spirotetramat, was significantly longer than that of the control batch. The activities of mixed function oxidase, acetylcholinesterase, carboxylesterase, and catalase increased significantly, and the rate of increase in enzyme activity was directly proportional to the increase in the concentration of spirotetramat. These results revealed that the parasitic ability of E. formosa decreased after exposure to spirotetramat at LC10, LC30, and LC50. This leads to a change in parasitoid control of pests, revealing the potential environmental threat of insecticide residues to nontarget organisms.

Effects of short-term extreme temperature treatment on the development and reproductive capacity of Encarsia formosa.

Encarsia formosa is a natural enemy of the invasive pest Bemisia tabaci and is known to be a dominant parasitic. The frequency and magnitude of climate extremes, particularly temperature extremes, have increased, which has put insect populations at risk. However, the effects of temperature extremes on E. formosa are not well understood. To examine the impact of short-term extreme temperature exposure on the development and reproduction of E. formosa, eggs, larvae, pupae, and adults were exposed to high/low temperature treatments (HLT25, HLT50, LLT25, and LLT50). Our findings indicate that the pupal stage of E. formosa exhibited the strongest tolerance to both heat and cold, while adults exhibited a weaker tolerance. The shortest egg-to-adult development period of 12.65 days was observed in E. formosa exposed to HLT50 treatment during the egg-larval stage. The parasitism peak of the adult stage was delayed by 1-6 days after exposure to extreme temperatures during the egg-larval stage. Conversely, the parasitism peak was advanced by 1-3 days after exposure to extreme temperatures during the pupal and adult stages. The eclosion rate, total parasitism, eclosion rate of the F1 generation, and adult longevity of the F1 generation were lower in the treatment groups than in the control groups. The F1 generation's development period was prolonged to 15.49 and 15.19 days after exposure to HLT25 and HLT50 treatments, respectively, during the egg-larval stage. The F1 generation's development period was shortened to 13.33 days after exposure to LLT50 treatment during the pupal stage. Male individuals appeared in the F1 generation after exposure to HLT50 treatment during the pupal stage, with females accounting for only 56.38%. Our results demonstrate that short-term exposure to extreme temperatures has detrimental effects on the growth and reproduction of E. formosa. In field biocontrol against E. formosa, the release of E. formosa should be avoided as much as possible when the ambient temperature is higher than 35°C or lower than 0°C. During extreme temperature conditions, timely supplementation and release of E. formosa population, along with ventilation and cooling in greenhouse facilities during summer, are necessary for better pest control efficacy.

Density and seasonal dynamics of Bemisia tabaci and its predators in different agricultural landscapes in South China.

Bemisia tabaci is the main pest of agriculture in many regions of the world. The resistance of whitefly to pesticides has increased as a consequence of the continuous irrational use of wide-spectrum pesticides. Thus, pesticides are no longer always effective as a long-term control method. The agricultural landscape can affect the occurrence of an insect population. The objective of this study was to clarify the occurrence of whitefly and its predators in tomato fields in different agricultural landscapes. Different landscapes are classified into urban, flower, water, and mountain landscapes by the principal component analysis method. In 2018-2019, whitefly had the longest main activity period and the lowest density in the flower landscape. The water landscape helped to maintain the highest densities of whitefly during the main activity period. Nine species of predators were sampled, and Nesidiocoris tenuis, Chrysoperla sinica, Menochilus sexmaculata, and Harmonia axyridis were the dominant species throughout the sampling season in both years. During the main activity period, N. tenuis had the highest density in all sampled landscapes. The density of the dominant predators was the highest in the flower landscape, and each natural predator had the largest temporal niche width in the 2-year sampling period. Bemisia tabaci, N. tenuis, and M. sexmaculata were highly synchronized temporally. The flower landscape showed satisfactory results in suppressing whitefly. Increasing the proportion of flowering plants and increasing the diversity of plant crops in the agricultural landscape can effectively reduce the densities of whitefly during an outbreak.

Effect of sublethal Spirotetramat on host locating and parasitic behavior of Encarsia formosa Gahan.

BACKGROUND: The use of chemical insecticides to control Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is widespread, although it might exert a sublethal effect on its dominant parasitoid, Encarsia formosa Gahan (Hymenoptera: Aphelinidae). To investigate the sublethal effect of spirotetramat on E. formosa, we observed the ability of E. formosa to locate and handle the host, oviposit and preen after exposure to sublethal concentrations of spirotetramat. RESULTS: After exposure to spirotetramat at LC50 , the response time of E. formosa to the volatile reached 223.40 s and was significantly prolonged. Only 56.44% of the wasps were attracted by the volatile and the insect crawled the slowest among all of the treatments. The averages of oviposition posture adopted and host handled by each E. formosa in 1 h decreased significantly to 1.79 and 1.27, respectively. At the sublethal concentration of LC10 , 94.59% of the wasps were attracted by the volatile and the insect crawled the fastest. The average of host handled by each E. formosa was 3.92, and the frequency of drumming while walking and drumming the host was 12.34 times per second and 12.30 times per second, respectively, demonstrating a significant acceleration in these abilities. CONCLUSION: These findings demonstrate that spirotetramat induced hormesis in E. formosa on exposure to its LC10 concentration and accelerated its host locating, host handling and frequency of antennae drumming. These findings could assist in balancing the chemical and biological control of B. tabaci and enhancing the efficacy of E. formosa as a biocontrol agent. © 2021 Society of Chemical Industry.