Seasonal occurrence and abundance of redlegged earth miteHalotydeus destructor(Acari: Penthaleidae) in annual pastures of southwestern Australia

Abstract

Seasonal occurrence and abundance of redlegged earth mite, Halotydeus destructor (Tucker), was measured by weekly sampling in grazed annual pastures near Keysbrook (1990Keysbrook ( -1992 and Narrogin (1991Narrogin ( -1992 in southwestern Australia. Mites were active for 27 weeks from the late autumn (May) to mid-spring (October), completing three generations at approximately 8 week intervals. The summer is spent as diapause eggs in the cadavers of adult female mites. In 1991 and 1992, active H. destructor was on average twice as abundant at Keysbrook (mean 11,300 mites/m 2 ), as at Narrogin (mean 6400 mites/m : ). Three times more eggs were laid at Keysbrook than Narrogin (mean 8500 and 2900 eggs/rrr respectively). Rainfall at Keysbrook was twice that at Narrogin, and temperatures were higher. We suggest that the rate of oviposition was less at Narrogin, probably because of resource limitation. The mature adult sex ratio was female biased, but was lower at Keysbrook (0.669) than at Narrogin (0.813). Neozygites acaridis, a fungal disease, was detected in less than 4% of the population, mainly in adult mites in late spring. Rainfall and temperatures were lower in the spring of 1992 than 1991 at both sites. Pasture was grazed considerably shorter in the spring of 1992, and numbers of H. destructor were lower, but numbers of eggs laid in the two years were similar. We suggest that active mite mortality was greater in spring of 1992, probably due to lower relative humidity. There were fewer adult mites in spring, and fewer diapausing eggs in summer at both sites in 1992 (36,600 diapause eggs/m 2 ) than in 1991 (148,000 diapause eggs/m 2 ). It is proposed that controlling mites in spring should lead to lower numbers of diapause eggs in summer and of mites emerging in autumn.

Predicting the timing of first generation egg hatch for the pest redlegged earth mite Halotydeus destructor (Acari: Penthaleidae)

“…Wallace suggested H. destructor has evolved strategies to avoid egg hatch following late summer or early autumn rainfall in the absence of declining temperatures to below 20.5°C. Ridsdill-Smith and Annells (1997) found no correlation between average weekly mite numbers and rainfall for the previous week, and similarly did not observe any particular pattern of association between egg hatch and weekly rainfall. Therefore, temperature and rainfall appear to act in concert to regulate egg development and subsequently egg hatch, and so should be useful in predicting hatch dates coming into a new season.…”

“…An anticipated advantage of the non-linear model, representing development rates more explicitly across the temperature range and particularly in the asymmetric post-optimum range, was not realised, perhaps reflecting a slower development in the field than expected from the laboratory data of Wallace (1970b). Ridsdill-Smith and Annells (1997) found a strong relationship between the hatching of first generation eggs and weekly maximum temperatures below 21.5°C. Changing the temperature thresholds for MDT in Models 3 and 4 generated considerable over-estimation of hatch dates against those observed, suggesting that, at least in WA, an upper threshold for egg development of 20.5°C MDT is appropriate.…”

“…Ridsdill-Smith and Annells (1997) found the first appearance of active mites in autumn was predicted by the week in which the mean weekly maximum temperature fell below 21.5°C, although the extent to which all mites hatched in the same week was not resolved. Hence, the MWTmax model (Model 1) used this threshold after a rainfall activation (described below).…”

“…The 5-15 mm rainfall model variants of the MDT (Models 2, 5 & 6) performed similarly well when the Narrogin (1991 season) dataset was excluded (Table 4). Of the five populations considered by Ridsdill-Smith and Annells (1997), Narrogin (1991 season) was characterised by unusually low egg numbers and egg hatch in autumn. A different predictive model performed well in south-eastern Australia.…”

“…Mite hatching of the first generation is thought to occur about 2 weeks after the first rain and cool weather in autumn (Ridsdill-Smith and Annells 1997). The challenge is to quantify the relationship between these weather variables and mite emergence, and then to use these variables to predict the median timing of egg hatch of H. destructor.…”