A successful application of the sterile insect technique and other strategies designed to eliminate large populations of insects relies on the efficient releases of competitive, sterile males into the natural habitat of the target species. As released sterile females do not contribute to the sterility of a population in situ, systems for the separation of male from female individuals are needed. This is especially key for vector-transmitting species like mosquitoes, in which only females bite and transmit diseases. While several genetic and transgenic approaches have been developed that permit male-female separation for some species, separation based on sexual size dimorphism continue to be a useful technique in the laboratory and in small to medium mass rearing settings for elimination of females at the pupae stage.
IR-604 Insect Sex Separator, designed to separate insect pupae and larvae into three size classes. In general, female mosquitoes of many species such as Aedes and Culex, but also Anopheles quadrimaculatus and A. albimanus are larger than males in the pupal stage. In addition to separating pupae by sex, the device can be used to separate pupae from the remaining larvae (see tray and rack rearing system). Separation sizes are set by adjusting the slit width of narrow glass plates against a glass backplate. Our model is thus based on the classical larval-pupal separator (narrowing glass plates, Fay-Morlan glass plate separator), but with a more convenient and efficient entry and washing/collection mechanism and three stages of separation - in particular allowing more accurate fractionation of populations with some overlap between sex sizes (e.g. smaller than usual females and larger males). In addition, because pupation is spread over time, it is necessary to separate pupae from the remaining larvae. Specimens are fed into the IR-604 from the top in an aqueous solution, which effectively transports the individuals either to the removable collection basket at each level or to a lower level (if smaller than the set slit width); the slope towards the basket is set by height adjustable feet. See the pdf document below for details/specifications of the IR-604 Pupae Sex Separator and a brief instruction manual. The IR-604 may also be used to separate pupae / larvae of other insect species if the individuals are i) in an aqueous solution and ii) have an explicit size dimorphism.
Size dimorphism in insect larvae and pupae refers to the presence of size differences between males and females of a species. In many insect species, such as A. aegypti (see example image), females are larger than males, a phenomenon known as "sexual size dimorphism". However, since not all larvae in the mass rearing trays become pupae at the same time, it is also necessary to separate the larvae from the pupae - so that the larvae can continue to be reared while the pupae are transferred to mass rearing cages for reproduction, or used for release in SIT, etc.
It is important to note that rearing conditions can influence the extent of size dimorphism in insect pupae. Nutrition is one key factor that can influence size dimorphism in insects - rendering a standardized feeding process imperative. Inadequate nutrition can lead to smaller body size, different feed application per tray will likely lead increased size heterogeneity and thus complicate sorting. Other environmental factors that can influence size dimorphism in insect pupae include temperature, humidity, and/or photoperiod. For example, low temperatures can slow insect development, while high temperatures can accelerate development. In addition, social factors such as crowding can also influence pupal size dimorphism. Crowding can lead to increased competition for resources, resulting in smaller or more heterogeneous body sizes.
In summary, rearing conditions, including diet, temperature, humidity, photoperiod, and social factors, can all influence the extent of size dimorphism in insect pupae. In order to exploit size dimorphism to separate insect larvae or pupae using a size separator such as the IR-604 (or other methods, as illustrated below), it is therefore essential to maintain constant and optimized (diet, temperature, etc.) rearing conditions, e.g. in standardized rearing tray and rack systems, leading to most homogeniously sized larvae and pupae (per sex).
Overall, the choice of method for sizing insect pupae depends on the insect species, particularly the size range between large and small fractions, the resources and equipment available, and the volume of pupae to be sorted. The effectiveness and efficiency of each method should be evaluated and compared to determine the optimal approach for the specific application. Vienna Scientific is ready to assist in the design of automated, effective sorting systems, please get in contact.
|Mechanical Separation and Size Dimorphism of Insects