Environmental effects on activity and honeydew collection by the weaver ant Polyrhachis simplex (Hymenoptera: Formicidae) when attending the mealybug Trabutina sp. (Homoptera: Pseudococcidae)

Abstract

The amount of honeydew collected by the weaver ant Polyrhachis simplex (Hymenoptera: Formicidae) was measured when it attended the mealybug Trabutina sp. (Homoptera: Pseudococcidae). Measurements were made in a relatively cool month (March; mean daily minimum and maximum air temperatures while the ants were active were 17 °C and 23 °C, respectively), a warm month (April; 18 °C and 28 °C) and a hot month (May; 20 °C and 35 °C). The Trabutina sp. were located on a tamarisk tree. No byres were made by the ants around Trabutina sp. clusters in March, initial byre stages were noted in April and many byres were found in May. In March, maximum ant activity was at midday, the hottest part of the day, whereas in April and May there were troughs of activity at that time. In May, the trough was more pronounced than in April and activity was high in the early morning while it was low in April. Honeydew fresh matter collected by the ants was significantly negatively correlated with air temperature and positively correlated with relative humidity. Dry matter collected was not significantly correlated with any of these two variables and showed no difference throughout the day or among months. Within months, the least amount of honeydew fresh matter was collected at the highest air temperatures and at the lowest relative humidities and, among months, the least was collected during the hottest month. It was postulated that evaporative water loss of the ants increased with high air temperature and low relative humidity. Activity and honeydew collection by the ants was highest in April. During March, 3372 ants entered the nest daily with 2900 mg dry matter of honeydew, during April, 8157 ants entered daily with 6933 mg and during May, 6207 ants entered daily with 5152 mg. This daily honeydew intake had an energy content of 51 1 kJ, 122.3 kJ and 90.9 kJ in the three months, respectively. Each Trabutina sp. attended by ants excreted 0–51 mg dry matter of honeydew per hour, whereas each unattended Trabutina sp. excreted 012 mg dry matter per hour.It was concluded that P. simplex used both behavioural and physiological mechanisms in coping with the environment. During cool months they increased their activity at the hottest part of the day. During hot months they reduced their activity during the hottest part of the day and they either remained in their nests, where the air temperature was lower and relative humidity was higher, or many ants remained in byres protected from much of the solar radiation. Those ants that were active during the hottest part of the day increased their evaporative water loss.