New Zealand has a rich fauna of sod webworms (Lepidoptera: Pyralidae: Crambinae and Scopariinae) which have been implicated in sporadic outbreaks of pasture damage. This study investigates sod webworms of south facing hill country slopes of some Waikato pastures. Four aspects are considered: taxonomy of the species present; some features of their biology; population dynamics of Eudonia sabulosella (Walker) (the most common species); and adult behaviour, especially flight. Taxonomy is detailed for male and female adults (with accompanying genitalia figures) and for eggs, larvae and pupae where available. Nine species are involved: Orocrambus flexuosellus (Doubleday), and O. vitellus (Doubleday) (Crambinae); Eudonia psammitis (Meyrick), E. sabulosella (Walker), “Scoparia” bisinualis Hudson, “S”. Diphtheralis Walker, “S”. halopis Meyrick, “S”. philerga Meyrick and “S”. submariginalis (Walker). A summary of meteorological data including soil moisture content, soil temperature, air temperature, relative humidity and rainfall is given for the main study sites. Life cycles and generation times with information on the duration of adult seasons are presented for each species. Habitats of eggs, larvae, pupae and adults are defined and selection of oviposition sites by female Eudonia sabulosella is investigated. Studies are made on food types, larval growth rates and adult reproductive maturity. Longevity of adults, mating frequencies and female fecundity (taken as the number of eggs found in dissected field collected females) are measured. Egg fertility and viability as well as egg incubation times are investigated. Life tables are presented for Eudonia sabulosella on hill country pasture. Key factor analysis is undertaken to identify major mortality factors. Parasitism by Aucklandella geiri (Ichneumonidae) and arthropod and avian predation on sod webworm larvae are studied. Pathogen infection of E. sabulosella by nematodes (some of which were Steinernema bibionis Wouts et al.) and a neogregarine protozoan (Mattesia sp.) is investigated. Effects of the protozoan on host size (body weight and length) and female fecundity are measured. Median lethal doses of pathogen are presented for larvae and pupae. Methods of protozoan transmission are discussed. Adult flight activity is interpreted in relation to air temperature, humidity, rainfall, barometric pressure, wind speed and direction, cloud cover, lunar cycle, reproductive state of the adult, time of day, species and sex. Susceptible pastures are indicated. An understanding of some of the factors affecting the sporadic (temporal and spatial) occurrence of sod webworm infestations has been gained.