Dose–Response and Temperature Dependence of the Mortality of Spider Mite and Predatory Mite Eggs Caused by Daily Nighttime Ultraviolet‐B Irradiation

Abstract

The two‐spotted spider mite, Tetranychus urticae, is an economically important agricultural pest. A novel physical control method involving daily nighttime UV‐B irradiation was recently developed for use in strawberry greenhouses. However, the overlapping of leaves after March prevents direct irradiation to T. urticae on the lower leaf surface, decreasing control effect. Excessive UV‐B irradiation causes leaf sunscald in winter. Therefore, optimization of UV‐B irradiance and a compensatory control agent are desired. Temperature may affect the survival of organisms exposed to UV‐B, although the temperature dependence of UV‐B damage is controversial. A phytoseiid mite, Neoseiulus californicus, is a prominent predator but vulnerable to a single UV‐B irradiation. We compared dose–response and temperature dependence of UV‐B damage between T. urticae and N. californicus eggs under daily nighttime UV‐B irradiation. Unexpectedly, N. californicus showed greater resistance to UV‐B than T. urticae, and the mortality was increased and decreased at low and high temperatures, respectively. This makes possible the application of UV‐B doses that are lethal for spider mites but safe for phytoseiid mites. Overall, we concluded that combined use of phytoseiid mites with UV‐B lamps is advantageous to spider mite management in strawberry greenhouses.

Mechanisms underlying the impact and interaction of temperature and UV‐B on the hatching of spider mite and phytoseiid mite eggs

“…In a previous study, egg hatchability under intermittent nighttime UV-B irradiation simulating the UV method drastically increased from 23.3% to 97.0% in T. urticae (0.0972 kJ m −2 day −1 ) and from 20.1% to 89.0% in N. californicus (0.27 kJ m −2 day −1 ), as temperature increased from 18 to 30°C. 8 However, in Method-1, the hatchability and photoreactivation efficiency in both T. urticae and N. californicus eggs decreased slightly, or did not change, after a single dose of UV-B irradiation exposure as temperature increased, except for N. californicus at 0.288 kJ m −2 . In that study, the hatchability of N. californicus eggs was positively correlated with temperature increase, regardless of Vis irradiation after UV-B irradiation; this positive correlation was likely caused by factors other than PER enhancement by temperature.…”

“…UV-B and Vis irradiation were administered using a UV-B lamp (6 W; Panasonic Co. Life Solution) and two halogen lamps (130 W; JDR110V-85WHM/K7-H; Ushio Lighting; separated by 22.5 cm), respectively, affixed to a shelf positioned at a height of 67 cm in a growth chamber, as described previously. 8,13,15,22 The growth chamber was illuminated with fluorescent lights (approximately 7000 lx) from 8:00 AM to 22:00 PM (14:10 h light/dark); the fluorescent lights were turned off as necessary for the experiment. UV-B irradiation from the UV-B lamp peaked at 310 nm (Figure S1), and its intensity was 0.16 W m −2 .…”

“…Neoseiulus californicus McGregor (Acari: Phytoseiidae) is a widely used biological control agent for spider mites 9–11 . Although N. californicus eggs are more vulnerable to a single dose of UV‐B irradiation than T. urticae eggs, 12 N. californicus shows higher resistance under the conditions used in the UV method 8,13 . Thus, determination of UV‐B doses that are lethal for T. urticae but safe for N. californicus is feasible, 8 having positive implications for biocontrol tactics.…”

Strawberry pollen as a source of UV‐B protection ingredients for the phytoseiid mite Neoseiulus californicus (Acari: Phytoseiidae)

“…As leaves grow and produce shade, spider mite survival may increase, especially in densely planted commercial greenhouses. Laboratory experiments have also revealed that rising temperatures decrease the mortality of spider mites on UV‐B irradiation 14 . These findings imply that the effect of the UV method on spider mites is dampened by biological and environmental factors in spring.…”

“…Although N. californicus eggs are more vulnerable to a single acute dose of UV‐B irradiation than are T. urticae eggs, 18 UV‐B vulnerability is reversed under the UV method. N. californicus eggs are more resistant to UV‐B radiation than T. urticae eggs 14,19 . Eggs are the development stage most sensitive to UV‐B irradiation 20,21 .…”