Reproduction is a biological process in which new individuals are produced by their parents. The inherent high reproductive capacity (rate) of certain insect and mite pests ensures that some individuals within an insect or mite pest population will survive natural mortality or other disturbances such as pesticide (insecticide or miticide) applications due to the excessive numbers produced. Therefore, due to their high reproductive capacity, it is critical to suppress insect and mite pest populations before they reach levels that result in plant damage. A high reproductive capacity can impact the success of plant protection strategies, including the application of pesticides and release of biological control agents (e.g. parasitoids and predators). For instance, more frequent applications of pesticides may be required and additional individuals (biological control agents) may need to be released at more frequent intervals to “keep up” with the reproductive capacity of certain insect and mite pests. However, more frequent applications of pesticides can lead to higher levels of pesticide resistance. Nevertheless, preventative approaches associated with plant protection strategies are essential to alleviate problems or avoid outbreaks of insect and/or mite pests.
Nearly all insect and mite pests associated with greenhouse-grown horticultural cropping systems have an inherent high reproductive capacity including: aphids, fungus gnats, leafminers, mealybugs, spider mites, western flower thrips and whiteflies. Aphids in greenhouses are predominantly females and do not have to mate to reproduce, which is called parthenogenesis. Parthenogenesis is asexual reproduction in which eggs grow and develop without being fertilized. Most aphid species in greenhouses give birth to live offspring or nymphs that are female. As such, new female nymphs can produce successive generations of female nymphs up to 10 days after being born. Moreover, aphids can reproduce for a period of 20 to 30 days. In fact, one aphid female can give birth to 60 to 100 live nymphs. Furthermore, many aphids can be produced in a short period of time. For instance, it has been reported that within five generations, a single green peach aphid (Myzus persicae) could give rise to 13,552,028 individual aphids. That is a lot of aphids!
Adult fungus gnat (Bradysia spp.) females can lay up to 200 eggs during their approximately 10-day lifespan. Vegetable leafminer (Liriomyza sativae) females live approximately one month and can produce between 200 to 300 eggs during their lifespan. Twospotted spider mite (Tetranychus urticae) females live up to 30 days and can lay up to 200 eggs in two weeks. Western flower thrips (Frankliniella occidentalis) females live up to 35 days and can lay up to 300 eggs (in plant tissues) during their lifespan with reproduction enhanced when females feed on flowers and pollen, which provide essential nutrients such as carbohydrates, proteins, sterols and vitamins. In fact, flower pollen can increase the reproductive capacity of western flower thrips. Adult female whiteflies can lay up to 20 eggs per day on the underside of plant leaves with females laying up to 250 eggs during a 30 to 45-day lifespan.
Temperature is an important factor that can influence the reproductive capacity of insect and mite pests, or number of eggs laid or offspring produced. In general, insect and mite pests have a higher reproductive capacity as temperatures increase, especially in spring and summer. However, temperatures below or above a certain optimum for reproduction can negatively affect various biological parameters. In fact, excessively high temperatures can decrease reproductive capacity. Citrus mealybug (Planococcus citri) females, for example, lay fewer than 100 eggs when temperatures exceed 86° F (30° C) but lay over 400 eggs at 64° F (18° C). Aphid reproduction can also be influenced by temperature. A study reported that the optimal temperature for three different aphid species was 72° F (22° C) with the total number of nymphs produced decreasing as temperatures increased. Twospotted spider mite females lay up to 84 eggs at 59°F (15° C); however, females can lay up to 130 eggs at 68 to 77° F (20 to 25° C). The optimum temperature range for western flower thrips female reproduction is 68 to 77° F (20 to 25° C).
Nearly all insect and mite pests associated with greenhouse-grown horticultural cropping systems have an inherent high reproductive capacity including: aphids, fungus gnats, leafminers, mealybugs, spider mites, western flower thrips and whiteflies.
Another factor that can influence reproductive capacity is plant nutritional quality. Plants that are over-fertilized, especially with nitrogen-based fertilizers, contain higher levels of amino acids (building components of proteins) that are the primary food source of insects and mites, consequently resulting in an increase in reproductive capacity. For example, aphids increase reproduction after feeding on plants that have been fertilized with excessive levels of nitrogen. Finally, body size, which is typically used to evaluate fitness, can affect reproductive capacity with larger females, in general, producing more young than smaller females. Typically, aphids that feed on plants with a high nutritional value are larger in size, and thus have a greater reproductive capacity.