Summer is usually the time of more intense insect and mite pest pressure, simply because temperatures are warmer and many major pests develop faster – or in some cases, just develop, which they do not do outdoors in a northern winter.
Mites and insects are “cold-blooded” animals that are affected by temperatures.
Feeding, reproduction, development time and survival are mostly dependent on temperature, but moisture, humidity, plant health, nitrogen level, and soil/potting mix contribute as well. Following are a few examples of greenhouse and nursery insect and mite pests that do best when temperatures warm.
Two-spotted spider mites
Two-spotted spider mites are warm-season mites, doing best when temperature are higher. It takes about 28 days to develop from egg to adult at a cool temperature range of 50°–68°F, but only about 8 days at a warmer temperature range of 77°–95°F. You can easily see that more spider mite generations in a given amount of time will occur at high rather than low temperatures.
Fortunately (or not, depending on your point of view), plant injury caused by two-spotted spider mites appears quickly on most plants and can be detected using a good scouting and monitoring program. Applications of effective miticides can then be made to stop the infestation’s spread.
Western flower thrips
Western flower thrips, or WFT, generally cause more problems on greenhouse-grown plants, but can also affect plants grown outdoors. WFT are warm – but not too hot – weather pests. The ideal temperature for development and reproduction is about 80°F. WFT development takes place between about 50 to 90°F. Thrips can survive temperatures lower than 50°F, but there is no development. Above about 95°F development again stops. With a warm temperature range of 65–95°F the egg to adult cycle is about 10 to 14 days.
At cooler temperature ranges, the egg-to-adult cycle extends to as long as 30 to 40 days. Growers have reported thrips infestations that seemed to appear overnight. Unless you believe in spontaneous generation, this seems unlikely. The probable causes for these “overnight” infestations are:
1. Movement into the crop from adjacent areas;
2. Favorable environmental conditions allowing the thrips – which were already there at low numbers – to increase rapidly.
Have I mentioned having a good scouting and monitoring program?
Leafminers develop from egg to adult in 14 days at 95°F to 64 days at 59°F. Other species have different lower and upper limits for development, but development trends are similar – warmer temperatures result in faster development. Leafminers generally do best when plants are high in nitrogen.
Primary Liriomyza leafminer injury is from the larvae feeding within leaves, making a narrow winding trail, or mine. Both greenhouse and outdoor crops can be infested. Adult leafminer flies puncture leaves for feeding and egg-laying, and the small white spots will indicate leafminer activity. Leafminers have a very wide host plant range.
Bemisia whiteflies are warm weather pests, with temperatures making a big difference in development times – 16 days at 86°F to 31 days at 68°F. A whitefly infestation will reduce a plant’s value, and high numbers can reduce plant growth or vegetable yields. Bemisia whiteflies can cause leaf spotting, white stem and bract deformation on poinsettia. Honeydew from whiteflies makes leaves and fruits sticky and is a substrate for black sooty fungus. Whiteflies can transmit many plant viruses affecting vegetable and ornamental plants.
Common aphids on ornamental crops are the green peach aphid (Myzus persicae) and melon/cotton aphid (Aphis gossypii), but numerous other species can be found on herbaceous and woody plants. Both melon aphids and green peach aphids will infest large numbers of host plants. Although many aphids generally do better at warmer temperatures, the best temperatures for development vary with the species. For example the chrysanthemum aphid develops best at 68°F, the green peach aphid at 73°F, and the melon aphid at temperatures above 75°F. When temperatures are above 86°F and the relative humidity is above 85 percent, green peach aphid longevity and reproduction is reduced – conditions that are likely to slow (or stop) reproduction of just about anything!
Managing summer pests
Weekly scouting of crops and the use of sticky traps for pests attracted to them are the most practical methods for detecting insects and mites and keeping tabs on how the management program is going. The bottom line here is that insect and mite generations are generally shorter at warm temperatures than at cool temperatures, and your management program needs to be adjusted accordingly.
Foliar sprays will need to be applied more often when it’s warm. However, on crops where it is known that a certain insect or mite will probably appear, it is acceptable to apply pesticides preventively – especially systemic products as drenches or granules, which need time to move up into the plants.
On outdoor crops, scouting and monitoring should be done as well, and there are methods to assist the process involving so-called plant phenology charts. Just as development of insects and mites depends on temperature, so does plant development. Clever and observant folks have long associated appearance of pests with development stages of certain plants. Other clever folks have put this information into charts that help with decision-making. Phenology charts are only accurate over a limited area – maybe a state or part of a state – so growers need to use information for their area. Again, this information will help if foliar spray applications are going to be used for control.
If the goal is to use a preventative management program with soil-applied, systemic products, applications need to be made before the appearance of the pest – sometimes well before – to minimize plant injury.