There are many factors that influence decomposition rates. Variation in temperature, can have a substantial impact by affecting decomposer growth and development. If you remember back to Module 1, we mentioned that arthropods are ectothermic. Which means that their metabolic and developmental rates are dependent on ambient temperature. The mechanisms that drive insect development, can be used to form predictions about patterns of development and geographical distribution. To do this, we must first understand how external factors influence development. Recall from Module 1 that insect post embryonic development, involves molting and metamorphosis. Juvenile insects grow in a specific manner during each stage or instar, separated by a molts in which the exoskeleton is shed and replaced by a new larger one. As we discussed in the first module, the exoskeleton protects the arthropod but also limits its growth. Through out development, different instars vary in terms of body weight, body proportion, and color. If a head capsule is present, its width can be a reliable measure of the stage of insect development. The specific number of larval instars required for development, varies with insect species. In some cases, the number of juvenile growth stages can also vary within a species depending on temperature, diet, larval density, and even sex. However, in most species, the total number of instars is genetically determined and remains constant regardless of diet or environment. The amount of time between successive insect molts is called the stadium, which is dependent on several external factors. Some insects will molt after they have reached a critical weight at a particular developmental stage. Many larvae have cuticle that can expand and stretch receptors on the abdomen will monitor body size and trigger molting. Interestingly enough, the larvae of some insects can be physically larger than the adult stage. This occurs in many holometabolous insects because they often need to store energy as a juvenile for the costly process of metamorphosis still to come. At the completion of the juvenile growth stages, the vast majority of insects go through metamorphosis which can result in subtle or dramatic changes in body form. These changes occur both internally and externally, although only the external changes are easily observed. We know that there are two types of metamorphosis and insects, incomplete and complete metamorphosis. There are also a few ancestral insects that do not go through metamorphosis and continue to molt and grow after reaching sexual maturity. Insects development follows a predictable temperature based pattern. The relationship between temperature and the rate of development, is well understood for some insect pest species. This can be used to predict the progression of developmental stages using degree-days. Degree-Days are measures of the accumulation of heat units over time. Specifically, they represent the number of degrees above a lower threshold temperature that accumulate in a 24-hour period. This lower threshold temperature is species specific and is the minimum temperature at which growth and development occurs. In addition to the lower threshold, many species also have an upper threshold temperature which is the highest temperature at which development still occurs. The lower threshold is more biologically relevant for the development of temperate species than the upper threshold temperature. It is used to set a zero point at which heat units begin to accumulate. The optimum temperature is that which the greatest percentage of development per day occurs. But all temperatures between the thresholds accumulate towards development. Insect species take a varying number of degree-days to develop through the various stages in their life cycle. As such, the ability to calculate degree-days is critical to estimate an insect's developmental rate. Given the relationship between temperature and insect development, we can create mathematical models that help determine an insect's developmental rate, based on the temperature of its habitat. These models are known as Degree-Day Models. A separate degree-day model is developed for each insect of interest. To create these models, insects must be reared at multiple temperatures. The number of days required to complete development at each temperature, can then be determined and used to calculate the percentage development per day at each temperature. All of this information is combined by a computer program to produce a species-specific degree-day model. Degree-day models have multiple applications for different fields of study. For instance, they are frequently used as part of pest management programs to help determine when the control methods that target different life stages will be most effective. They can also be used to predict when pest damage is most likely to occur, by comparing ambient temperatures with models of how many degree-days must accumulate for that species to reach a particular life stage. A real life example of this is the spruce budworm in which the fourth and fifth instar larvae, are most susceptible to insecticides. The spruce budworm has a lower threshold of six degrees Celsius and requires 235 degree-days to reach the fourth instar. The degree-day model for the spruce budworm incorporates this information to determine when biological or chemical insecticides will be most effective. Along with ambient temperature data, the model can be used in pest management to determine when the 235 degree-days have accumulated, so that controls can be applied when the majority of larvae in an infested stand will be in the fourth instar. Degree-day models are also used extensively in forensic entomology to determine the developmental stage of larvae. Learn all about this in the next lesson. Because of the strong influence of temperature on development in insects, the number of generations per year or the voltinism can vary across the species geographical range. Warmer temperatures allow for faster development and so the number of generations produced annually in some species, varies with temperature. Species that produce two generations per year are bivoltine and those with more than two generations per year are considered multivoltine. Take for example, the mourning cloak butterfly. This species of butterfly has a broad geographical distribution that extends across North America and Northern Eurasia. Populations of these butterflies in the Southern part of the range are multivoltine and typically produce up to four generations per year. Populations in the Northern regions of its distribution, however, produce only one to two generations per year. Some insects have an obligatory diapause during development, this results in a fixed voltinism regardless of temperature, usually of one generation per year. The gypsy moth for instance, has eggs which undergo an obligatory diapause over the winter months. The embryos complete development but do not hatch until the following spring. This diapause always occurs. So gypsy moths will always produce one generation per year. Many coprophages and necrophages insects are multivoltine, because their development must be fast to take advantage of their ephemeral resources, dump and carrying. This fast development is associated with the production of multiple generations per year. Now, you should have an understanding of how temperature can influence insect growth, development, and generation times. Well, many factors contributed to this. Temperature is the most common variable we use to predict insect growth and activity. In the next lesson, we will explore how information about insect development can be used in forensic investigations.
