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Controlling the many insects and mites of summer
by Dr. Richard Lindquist, OHP Senior Technical Manager

“I sing of brooks, of blossoms, birds, and bowers: Of April, May, of June and July flowers” Robert Herrick (1591-1674)

Yeah, right. It might have been easy for Robert Herrick to sing about all of this, but he probably was not in the greenhouse or nursery business. Summer is usually – but not always - 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 and soil/potting mix contribute as well.

Following are a few examples of how temperatures affect some major greenhouse and nursery insect and mite pests. Some like it hot and some do not. Others don’t seem to care much. File this information with your possible topics of conversation with people you don’t like very much.

Two-spotted spider mites and Spruce spider mites
Two-spotted spider mites (Figure 1) are warm season mites, doing best when temperature are higher. It takes about 28 days to develop from egg to adult at a cool temperatures 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.

To further show how this works, consider the following: If we begin with one spider mite female, after one month at 60°F there will be 20 mites. After one month at 70°F there will be about 12,000 mites. Finally, after one month at 80°F there will be about 13,000,000 mites! Some poor graduate student probably had to count them.

Fortunately (or not, depending on your point of view), plant injury caused by twospotted 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.

On the other hand, spruce spider mites (Figure 2) are cool season mites, doing best in spring and fall,when temperatures are below the mid 70’s F – but they will remain active until temperatures consistently get to the upper 80’s F. Below that high temperature threshold the life cycle ranges from about 11 to 24 days. When the hot temperatures arrive, spruce spider mites basically do an overwintering in reverse: they oversummer, laying eggs and becoming dormant until cooler temperatures return in the fall. Of course, if the summer temperatures remain cool – a rare occasion these days – the mites remain active.

Unlike two-spotted spider mite feeding injury, plant damage from spruce spider mite feeding on conifers does not appear right away, and injury that appears in June or July may have resulted from feeding in the spring –or even the fall of the previous year. This again points out the need to have a good scouting and monitoring program for this pest on susceptible plants.

Western flower thrips
Western flower thrips, or WFT, (Figure 3) are mostly 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 90ºF. Thrips can survive temperatures lower than 50ºF, 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?

Potato leafhopper
Potato leafhoppers (Figure 4) are examples of insects that survive the winters in the Gulf States and move up to northern states on wind currents as temperatures rise in the spring – usually arriving in May or early June. In this respect these insects are similar to the snowbirds that winter in the South and summer in the North – except that the leafhoppers that summer in the North never make it back south. Some southerners might think this should apply to human snowbirds as well, but that’s another matter.

Potato leafhoppers develop from egg to adult in about 20 days, and females deposit their eggs over about 30 days. There are at least two generations per year in the North, and, of course, many more in the South. Again, if you are a nursery operator, knowing when the leafhoppers appear will help reduce injury to your trees.

Managing summer pests
Let’s see. I believe that scouting and monitoring have been mentioned. In greenhouses, 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 preventatively – especially systemic products as drenches or granules, which need time to move up into the plants.

Outdoors, 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. I should point out that plant 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.