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Aphids & Thrips
Early season aphid and thrips populations
May 2006
This has been a good year (bad for growers though) for aphid and thrips populations. These two pests have been in greater than average numbers since early in the season, mostly do to the weather.
APHIDS were first seen by growers on their strawberry crop, then on pea and now on cantaloupe, watermelon and other cucurbits. The very dry spring we had is probably most responsible for the high numbers of these pests. Frequent rain will reduce the number and duration of aphid flights, so they do not disperse as far or frequently. Rain storms also beat the leaves of plants and will dislodge aphids essentially dooming them. And finally wet, cool or warm conditions also will increase the likelihood of infestation from entomophagous fungi such as Neozygites fresenii, Beauveria bassiana or Metarhizium anisopliae. Aphids are especially susceptible to many of these fungal pathogens.
Some fungi, such as the Entomophthora and related species, are fairly specific with regard to the insects they infect; others, such as Beauveria spp, have a wider host range. Most of these fungi do not have a common name, but a few are sometimes referred to as "the aphid fungus". These fungal diseases are more effective than most of the insecticides used for controlling large populations of aphids. Growers probably could not effectively control aphids without these naturally occurring fungal diseases. Once active in a population of aphids the fungal disease spreads quickly, creating an epizootic. Once an aphid population collapses due to the fungal epizootic, populations tend to remain low for the remainder of the year in that system. Unfortunately, epizootics did not occur to any great extent in aphid populations this spring as the aphids began to leave their overwintering hosts and move into vegetable and cereal production fields.
Aphids that have been killed by the fungi often appear in the peculiar position of standing on their heads with their mouthparts inserted in the leaf. Aphids that have just died are light gray and are covered with a thin layer of fungal spores. Aphids that have been dead for a few days often become covered with a thick olive colored wooly fungal mass (Fig. 1). This wooly mass, however, is not the original fungus that killed the aphid, but a secondary fungus that grows on the dead aphids. The presence of this wooly, olive colored fungus (aphids often times cannot even be seen because the fungus is so thick) is still an indication that an aphid fungus was present.
Aphids become infected when they brush against sticky spores that are produced by the fungus on the surface of a leaf. These spores are about ¼ mm in height. The spores stick to the passing aphid, germinate and penetrate its body. This is much more likely to occur under long periods of very humid or moist conditions. The fungus then grows in the hemolymph (blood) of the aphid, killing it and producing fungal spores that are discharged into the air to repeat the cycle. It takes about three days for an aphid to die after it becomes infected.
Besides dry weather these fungi can be reduced by frequent early season applications of some fungicides. So it is important to apply fungicides only when needed.
Even with this increase in the aphid population growers have sometimes treated for aphids unnecessarily early in the year on crops such as strawberry and later in the season on melons. This is because aphid populations that developed on cereal crops such as rye, wheat, barley, etc, early in the spring took wing from these crops in large numbers when the cereals were cut or started to become less favorable hosts. These aphids often times then ended up on vegetables and strawberries. Growers would see these large numbers of aphids suddenly appear and would think they needed to treat—but they didn’t. Some of the most common cereal aphids in our area are the English grain aphid (Sitobion avenae), Bird cherry-oat aphid (Rhopalosiphum padi) and Green bug (Schizaphis graminum). These aphid species will not colonize vegetable crops to any extent. Most will leave after landing on or tasting the plant, some will try to feed, but will not be successful in gaining any nourishment and a very few might be able to feed and reproduce a few weak individuals. Cereal aphids cannot survive on vegetables such as strawberry, cucurbits and peas. However, there are other common aphid species that do well on vegetables. The three most common aphid species in the mid-Atlantic that survive well, have a large host range and are pests of vegetables are the green peach aphid (Myzus persicae), melon aphid (Aphis gossypii), and potato aphid (Macrosiphum euphorbiae). In order to know if treatment is warranted it is necessary to identify the species of aphid that just landed on your vegetables. In general any aphid species that builds up on cereals will not colonize a vegetable crop.
I will be going over how to tell some of the most common cereal and vegetable aphid species apart at future winter meetings. For now the following web sites have good pictures and descriptions of the most common aphids a grower would encounter in their vegetable or cereal crop in the mid Atlantic area:
http://www.gov.mb.ca/agriculture/crops/insects/fad05s00.html
http://www.ipm.ucdavis.edu/TOOLS/KEYAPHIDGRAIN/beginkey.html
THRIPS are those pesky small yellowish-orange insects the size of metal filings that like to feed in the flowers of most vegetable plants. They started in strawberries this year and then moved on to peas, cucurbits, tomatoes, peppers and anything flowering. There are several species of vegetable thrips with the most common being the Eastern flower thrips, Frankliniella tritici, Tobacco thrips Frankliniella fusca, Western flower thrips, F. occidentalis and Onion thrips Thrips tabaci. F. fusca, F. occidentalis and T. tabaci are the three thrips species most likely to transmit tomato spotted wilt virus, TSWV. Luckily this has not been a problem so far this year. Flower thrips concentrate their feeding within the flower, where they consume pollen and some nectar. This feeding habit makes them difficult to control at times.
Thrips can be seen in just about any flowering plant this year as the mild winter coupled with the dry spring increased their numbers early in the year. One or two applications of a pyrethroid or spinosad applied with enough water (80-100 gal/a) should control most thrips infestations. Spraying more than 5-6 times for thrips control in a 4-5 week period will usually lead to an even worse thrips problem. This is because the sprays will reduce eastern flower thrips and most other thrips species, but usually not western flower thrips that are often resistant to many of the pyrethroids and organophosphates. Once the thrips population consists mostly or entirely of western flower thrips it may not be possible to get the population under reasonable control. This is due to the spray treatments reducing if not eliminating all natural enemies. With no natural enemies the grower becomes totally dependent upon insecticides for thrips control.
It is not possible to identify thrips to species in the field. Thrips color, size, markings will not separate the different species from one another to any extent. It is best to assume you have mostly eastern flower thrips (although if you grow onion or tobacco you also will have onion and tobacco thrips) and only a few western flower thrips. If your usual thrips control sprays do not work and thrips seem to getting worse you should switch to spinosad. Spinosad will control western as well as eastern, and other flower thrips. Spinosad will take a few days to reduce the thrips population. It exhibits translaminar movement, which means it is absorbed into the leaf where it is active and readily available to kill thrips or caterpillars that feed on the leaf tissue. This gives spinosad a fairly long residual of seven days or more even with rain.
It is not possible or necessary to eliminate all thrips from a crop. Thrips feeding on strawberry blossoms can cause stigmas and anthers to turn brown and wilt prematurely, but this feeding damage is usually minor. Thrips also feed on tissues around the achenes of very small, developing fruit, which later appears as bronzing damage. It should be noted that bronzing is also caused by adverse weather conditions and other factors. Thrips feeding in strawberries does not cause catfacing, which is caused by lygus bug feeding. There is no good threshold for thrips in the flowers of strawberry. This is because large numbers of thrips have been found in strawberry flowers with no resulting damage to fruit. Growers should become concerned however, when 5 thrips are found per strawberry flower. Once fruit forms eastern flower thrips should cause little if any additional damage. Flowers of tomato and pepper can tolerate 5 thrips/flower with no fruit developmental problems. Squash flowers can tolerate 5-10 thrips/flower with no effect on fruit quality. Thrips do not cause the white blotching on tomato fruit. These white blotches are caused by stinkbugs feeding later in the season. Thrips populations need to be monitored in vegetable flowers, but not over reacted to. If a grower wishes to know what species of thrips they have they can send a sample of 20-30 infested flowers placed in a plastic zip-lock bag with just a little alcohol (rubbing alcohol is fine) to: Jerry Brust, LESREC 27664 Nanticoke Rd, Salisbury, MD 21801-1922.
References
Fig. 1 found at: http://muextension.missouri.edu/explore/images/ipm1025art61.jpg
For more information, contact Dr. Gerald Brust
Last updated: 03/10/2009