Some aspects on the biology of Deroceras panormitanum and Derocerus reticulatum with special emphasis on effects caused by some agricultural chemicals : a thesis presented in partial fulfilment of the requirements for the degree of Masterate of Science in Zoology at Massey University

Abstract

Two species of slugs Deroceras panormitanum ( Lessona & Pollonera, 1882 ).and Deroceras reticulatum ( Muller, 1774). were chosen as a subject for investigation. In part 'A' the fecundity and longevity of D. panormitanum was observed at four different locations, three at constant temperatures, and the fourth in Shade-house, which was the control site. A seasonal effect was evident in Shade-house, more eggs being oviposited in Spring and Autumn than in Winter. No such effect was observed with the constant temperatures. The number of eggs per cluster was 22.5 in Shade-house, 23.2 at 16 °C, 22.9 at 24 °C, and 18.5 at 5 °C. The average number of clusters oviposited per slug was 1.3 in Shade-house, 1.4 at 16 °C, 1.1 at 24 °C and 0.4 at 5 °C. The average number of eggs laid by each slug in this part of the experiment was 28.7 in Shade-house, 31.8 at 16 °C, 25.2 at 24 °C, and 7.7 at 5 °C. The optimal condition for slugs to oviposit was at 16 °C. Thirteen D.panormitanum and fifteen D. reticulatum that hatched on the same day were kept in the laboratory till natural death ensued. The average number of eggs per cluster for D. panormitanum was 15.6, and for D. reticulatum 15.4 . The number of clusters per slug was 3.8 for D. panormitanum and 3.5 for D. reticulatum, and the average number of eggs oviposited per slug was 59.8 for D. panormitanum, and 53.4 for D. reticulatum. The effects of Temperature, Humidity, and Evaporation-rate, was correlated with oviposition rate. An increase in temperature and evaporation-rate showed a positive correlation, with an increase in oviposition rate. Humidity has a negative correlation with oviposition rate. Significantly more eggs hatched from D. panormitanum 59.9 %, than of D. reticulatum 53.0 % , under laboratory conditions. The time taken for eggs to hatch is temperature dependent, taking for D. panormitanum an average of 33.7 days for Shade-house, 20.9 days at 16 °C, 16.5 days at 24 °C, and 103.4 days at 5 °C. The average number of eggs hatched for D. panormitanum in Shade-house was 38.3 % , at 16 °C 37.0 %, at 24 °C 32.1 % , and at 5 °C 25.4 %. In the laboratory D. panormitanum's average life-span was 171 days, and D. reticulatum 151 days. At the four temperatures D. panormitanum survived for an average of 32.7 days in Shade-house, 23.2 days at 16 °C, 16.8 days at 24 °C, and at 5°C for 63.1 days. In part 'B' eightythree biocides were tested against the slug species D. panormitanum and D. reticulatum. These included 16 fungicides, 16 insecticides, 26 herbicides, and 2 molluscicides, at the maximum rates as specified by the manufacturer. Five fungicides, two insecticides, five herbicides, one molluscicide, and seven of the combinations showed high ovicidal activity. Five insecticides, one molluscicide, and ten of the combinations showed high toxicity when ingested, and two insecticides,one molluscicide, and six of the combinations were highly effective when used as a surface spray. Metaldehyde and methiocarb were effective in all three treatments,and phorate was a good bait and contact molluscicide. Dazomet caused a reluctance by slugs to cross the treated area to obtain food, and as a result died of starvation in the refuge area. The effects of all two possible combinations of three herbicides and three insecticides could not be determined from a knowledge of their individual properties. Each reacted in an undetermined manner according to their combined properties.