Effective Period for Control of the Brown Spiny Field Mouse (mus Plantythrix) in Dry Land Crops

Information on breeding aspects of rodents is important to ensure control program timing necessary for effective pest management. Hence, studies on breeding aspects of the brown spiny field mouse (Mus platythrix), a known rodent pest in dry land crops were investigated including environmental factors influencing reproduction, from regular monthly collections made during the years 1990 and 1991. Field trapped rodents were classified to derive their population structure—percent juvenile, pre-pubertal and adult including prevalence of pregnancy—based on body weights and sexual conditions. The stage of pregnancy and sperm motility were confirmed by autopsy studies. The data were also analyzed to calculate the annual breeding productivity. The results indicate that the brown spiny field mouse experiences a seasonal reproductive cycle of active breeding during the period June to February followed by sexual quiescence during the period March to May. The peak reproductive activity seen during September (55%), October (79%) and November (62%) coincides with peak abundance of heavier males and females in the population. A litter size of two (during June) to eight (during October) also coincides with peak prevalence of pregnancy. The adult male to female ratio is 1.0:1.1, respectively. The annual breeding productivity is calculated M be 54 young/female/breeding season. Further prevalence of rainfall, low temperature, short day length, and higher relative humidity seen during July onwards promoting green vegetation are conducive to reproduction. Hence, it is inferred that the period March to May is suitable for timing the rodent control programs in dry land crops against the brown spiny field mouse, when the field population is at base considering the cost-benefit ratio and operational aspects. Proc. 16th Vertebr. Pest Conf. (W.S. Halverson& A.C. Crabb, Eds.) Published at Univ. of Calif., Davis. 1994. INTRODUCTION Studies on breeding aspects and the reproductive cycle of rodents are important in understanding rodent breeding potential and in timing rodent control programs to achieve more cost effective management. Among the rodent pest species of economic importance, the members of the genus Mus are relatively less understood in relation to crop loss (Rao 1992). In India, Ellerman (1961) recognized six species under the genus Mus. Both Mus platythrix and Mus booduga have been recognized as rodent pests in agricultural crop fields of ragi, maize, groundnut, tomato, cucumber, and also occur in paddy in irrigated fields of Kamataka, South India (Prakash and Mathur 1987, UAS 1990). The brown spiny field mouse (Mus platythrix) is distributed throughout the country and occurs mostly in the borders of cultivated fields, preferably in the rain fed agricultural fields. Chandrahas (1974) outlined the ecology of the brown spiny mouse together with breeding aspects in connection with studies on the wild rodent plague reservoirs in and around Kolar. Rao (1981) also conducted similar studies on the reproductive biology of the brown spiny field mouse around Tirupati, falling under the semi arid zone. With this backdrop, the present studies were conducted under the national rodent control program, mainly to understand the reproductive cycle of the brown spiny field mouse so as to provide definite information and recommendations to the farming community to time/focus the rodent control programs for effective field-scale operations. METHODS Brown spiny field mice were collected from the dryland agricultural fields in the area of Bangalore. Usually one crop was harvested in these dry lands under rain fed cultivation. Vegetables, such as tomatoes, cucumbers, etc., were grown in some areas under protective irrigation. The brown spiny field mice were collected every month by live trapping, and sometimes a few burrows were dug up to collect the juvenile mice. They were removed live from the traps and immediately killed with ether in the laboratory. They were classified using the following criteria. 1. Juvenile/Immature: smaller body weight (5-10 gm); mammary teats not visible, vaginal orifice imperforate. 2. SubAdults/Pre-pubertal: body weight (10-20 gm); at autopsy juvenile conditions of the gonads and reproductive sex accessories, vagina imperforate and mammary teats undeveloped. 3. Mature/Adults: larger body weight (20 gm and above); at autopsy ovary conspicuous with follicles, uterus edematous in females, scrotal testes and presence of motile sperm in the cauda epididymis in males. 4. Pregnant: body weight 20 gm and above; swollen abdomen and conspicuous mammary teats (glands), embryos detected by palpitating the abdomen, vagina imperforate. 5. Quiescent: reduced body weight (19-22 gm); mammary teats regressed, vagina imperforate; at autopsy uterus shows placental scars and sex accessories in male and female regressed.