Bionomics of the potato tuber moth,Phthorimaea operculella(Lepidoptera: Gelechiidae), at Pukekohe

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Abstract
The life cycle, behaviour, and population dynamics of Phthorimaea operculella Zeiler were studied in the laboratory and in the field at Pukekohe, New Zealand. Populations displayed typical inverse relationships between stadial duration and temperature when reared on potatoes in cabinets which reproduced mean diurnal summer (28°c, 8 h; 17°, 16h), spring/autumn (22°, 8 h; 11°, 16h), and winter (17°, 8 h; 6°, 16 h) temperatures at Pukekohe. Ranges of stadial duration were: egg, 5–25 days; larva, 18–60 days; prepupa, 2.5–18 days; pupa, 8–23 days; adult (pre-oviposition), 2–23 days. For a complete generation a mean of 8471 hour-degrees above 10°c was required. A 50% increase in the hour-degree requirement of the prepupa in the ‘winter’ regime indicated a lowering of intrinsic metabolic rate, and provided evidence for the prepupa being the main overwintering stage. The ‘winter’ regime bordered on the lower temperature threshold for adult function. The adult sex ratio of approximately 0.5 was not affected by temperature. Life tables constructed under the three temperature regimes showed Type II to Type III mortality curves for P. operculella. Within the cardinal limits for development, temperature-related mortality was a relatively constant function of time. Potential population multiplication rates were estimated to be 20.4, 39.6, and 0.98 times per generation for the ‘summer’, ‘spring/autumn’ and ‘winter’ regimes respectively. Behavioural studies in the laboratory showed that flrst-instar larvae were prevented from reaching tubers by as little as 0.5 cm soil coverage, and increased soil moisture enhanced this inhibition. Larvae forced to leave depleted foliage were unable to locate and infest developing tubers covered with more than 1 cm of soil. Accessibility of developing tubers to ovipositing adults was an important factor in tuber infestation in the field. Under normal cultivation methods 45% of the potato plant rhizosphere was found to be at high risk of infestation by potato tuber moth. This could be reduced to 25–39% by greater soil coverage, and to 7% by increasing soil moisture. The annual period of leaf mining activity regularly occurred between mid January and mid May, during which time larval densities were usually greater than 5 larvae/plant, rising to a peak of 50–150 larvae/plant during February. Foliage mining activity continued throughout the year at low levels. Seasonal variation in population density at Pukekohe was regulated largely by positive temperature influences. Adult population peaks closely coincided with those of the larval foliage mining population, but adults were not present over winter. Flight activity increased with increase in temperature, and was inhibited by wind speeds greater than 3 m/s. Adult sex ratio was positively correlated with population density of the preceding generation. The introduced parasite Apanteles subandinus Blanchard was well established at Pukekohe. Although parasitism levels rose to 45% in the 1973 overwintering tuber moth population, this was insufficient to prevent the occurrence of a large and damaging pest population the following summer.

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