Population Dynamics of Maladera matrida (Coleoptera: Scarabaeidae) in Peanut Fields in Israel

The place of origin, time of occurrence, direction of invasion, and maturation status of Maladera matrida Argaman immigrating into peanut fields were determined. The timing of massive invasion of beetles was correlated with that of the wheat harvest in nearby fields. More than 80 beetles per trap were collected in the margin of peanut fields adjacent to wheat fields during the night after wheat harvest. Wheat fields rotated with peanuts and the citrus grove supported the highest population of immature stages during the winter months, even though all sites rotated with peanuts and adjacent winter crops had a similar population size at the 1st sampling date (≈3 grubs per soil sample). Almost all females trapped at the first 2 wk of trapping (immigrants) were pre-oogenic, with undeveloped ovaries, indicating a high reproductive potential. The life cycle of M. matrida consisted of 2 generations per year, with overwintering grubs emerging in April, 1st instars appeared in the soil sample 21 d later and 1st instars of the 2nd generation were observed in the soil sample 57-70 d after the appearance of 1st instars of the 1st generation. Grub populations in the center of the field observed 6 wk after 1st grubs observed in the soil samples in the margin of the fields. The sex ratio of trapped beetles was female-biased (63%), whereas a 1:1 sex ratio was found for active beetles on host-plant foliage. This inconsistency of the adult sex ratio is a result of the differences in the behavior of males and females. Females are attracted to males and fly for longer times and distances than males, therefore more females were caught in traps but 1:1 sex ratio was observed on foliage. The results of this study strongly recommend placing food-baited traps on the periphery of peanut fields at sprouting before wheat harvest for monitoring the population size. At the same time a control means should be applied only at the field margins.