Aphids - Early Pests of the Season

Wow, what a winter! Here in Maryland we had record-breaking snowfall with over 76 inches from two storms. The East Coast has experienced one of the coldest Februaries in over 50 years. If you’re a cut flower grower using a greenhouse to start cut flower transplants, this past winter you spent a lot of time knocking off snow off it, spent lots of money to heat it, and tried real hard to melt the remaining snow so the weight did not collapse the structure. Some desperate growers cut the plastic on their greenhouses to prevent snow load damage and just went without greenhouse covering for February. The worst part of these big storms was that after the snow hit the East Coast it hung around for a very long time, melting ever so slowly. Most people were glad to see February end, move into spring, and get plants going for the 2010 season.

If you’re using a greenhouse to get an early jump on the season, better get your rear in gear and set up a regular monitoring program so insects and disease don’t eat into your profits. Greenhouse and high tunnels are great and really empower a cut flower grower, but with great power comes great responsibility (quote from “Spider-Man 2”, I think). Bugs love greenhouse plants as much as you love to grow plants. You don’t need to live in fear of insects consuming your plants if you conduct regular monitoring and combine this with good sanitation practices.

Good (Green)house Keeping

Ideally, you have completely cleaned out the greenhouse, removing all old carryover plants, pet plants, and weeds. It is an excellent sanitation practice to sweep debris or shop-vac plant litter. If you have a spun-down polypropylene weed barrier on the greenhouse floor then a power washer can be used to clean the floor. Most males like to power wash things since it involves equipment that sprays a lot of high-pressure water.

Afterwards use a hand sprayer to apply a disinfectant such as a quaternary salt (Greenshield) or Clorox (1 part Clorox to 10 parts water) to benches and growing areas. Notice I said “Clorox”. This is because Clorox is the only sodium hypochlorite that has an EPA label as a disinfectant for use in greenhouses. Store brands do not have this EPA labeling.

Do not leave any weeds on the greenhouse floor because these will serve as reservoirs for insects and mites that will easily migrate onto your cut flower transplants. This is where you get your spring exercise, bending over to pull weeds. You can use glyphosate to kill the weeds if the greenhouse is completely empty and you have the vents closed. Hand weeding is usually not too much work unless you let weeds get out of hand during the fall/winter. Besides, everyone likes skinny, good-looking cut flower growers. If you don’t want to be slim and attractive, there are chemical weed options which include use of Diquat (Reward), or short-chained fatty acids (Scythe). Reward and Scythe just burn back the tops of the weeds and are not as systemic as glyphosate.

First Bugs of the Season

One of the first pest samples we usually receive from cut flower growers in the spring is aphids. Many species of aphids are lurking out there, but it is usually one of the “group of four” that we see at the lab. This very trendy and popular group includes green peach aphid, melon aphid, potato aphid and foxglove aphid. These four species can reproduce rapidly in a greenhouse, causing you much frustration and anxiety. The key is detecting them early, and taking action as soon as you find them. In most cases, aphid populations build up as little epicenters in your greenhouse. Find these epicenters and deal with them quickly before they get a chance to spread.

Aphid populations can virtually explode in greenhouses. When greenhouses become packed with tight spacing of plants, as is often the case in spring, populations of aphids can build and spread rapidly. While a grower concentrates on producing plants and preparing fields, aphid buildup may go unnoticed until the situation is out of control.

In greenhouses, most aphid species are able to reproduce asexually (parthenogenesis), with females giving birth to approximately 100 live female offspring (viviparous). This high reproductive capacity and short development time means that more frequent applications of insecticides may be required, thus increasing the probability of aphid populations developing resistance.

Certain species of plants are highly susceptible to aphids and should be checked frequently in spring. Plants that are big draws for aphids include sage (just about any sage is a magnet for aphids), dianthus, coleus, basil, snapdragons, zinnias, and any of the ornamental peppers grown for cut stems.

Monitoring is Key

Early detection is critical to prevent an aphid population from spreading throughout the greenhouse. When using biological control organisms, it is important that the release of natural enemies be initiated early in the cropping cycle. What is just as important is which plants tend to be susceptible to aphids so that potential problems can be avoided when a crop is moved into a greenhouse.

During the feeding process, aphids extract between 11% and 12% protein, and large quantities of water and sugar from plants. The water and sugar initially ingested is excreted from the aphid’s anal opening. This excrement, which covers leaves, is a sweet, sticky substance called honeydew. Honeydew is an excellent growing medium for black sooty mold fungi, which can cover foliage and flowers. Look for shiny foliage in the greenhouse since it is something that you can easily see (even if your eyes just are not what they used to be when you were a novice in the cut flower business).

Aphids shed their old skin (cuticle) as they progress through development. This process is called molting. The old (cast) skins, which may be present on leaves, appear white.

Aphids differ in their distribution on plants and dispersal capability, which can influence monitoring procedures. The melon aphid tends to be located in the plant interior, aggregated on the stems and flower buds, whereas the green peach aphid tends to be less concentrated, but typically located on the terminal growth of plants. They may blend in with the foliage due to their light green color. Green peach aphid is also more mobile than melon aphid, often dispersing to adjacent plants. The potato aphid congregates in large numbers on plants, and the distinct strip extending down the middle of body is noticeable.

All cut flower growers should invest in a good 10-20 X hand lens, and use it to examine foliage and stems closely for presence of aphids.

When populations of aphids build up to very high levels on plants, female aphids will start to birth out aphids with wing buds. Fully formed wings on adult aphids can be twice the length of the body of the insect. The winged forms can disperse in your greenhouse, spreading the populations to new plants. If you reach this stage it can be a tense time in your growing areas, and you better take action quickly or things will get out of control rapidly.

Yellow sticky cards will capture winged aphids. Although yellow sticky cards may indicate the presence of winged forms of aphids, it is still important to inspect plant foliage at least weekly to detect aphid populations early enough, so that the appropriate controls may be implemented.

Control Options

Aphidius colemani is a parasitoid we have used with success in Maryland growers’ greenhouses. It attacks green peach aphid, but it is more successful parasitizing melon aphid (Aphis gossypii). Females of both species lay eggs into aphids; the eggs hatch into larvae that consume the internal contents of their host. As the parasitoid larvae mature, the aphid body expands and turns brown to tan. The emerging adult parasitoid creates a round exit hole in the dorsal (top) side of the aphid’s abdomen. Emerging adults will mate and then females of the next generation will search out and parasitize any aphids that are present.

Aphidoletes aphidimyza is a predatory midge that attacks over 50 species of aphids including melon aphid, green peach aphid, and potato aphid. The larvae stage feeds on aphids whereas the adults do not feed. Adults feed on honeydew produced by aphids. Adult females locate aphids using visual cues and olfaction (odors), and then lay eggs adjacent to aphid colonies. A single larva consumes 10 to 25 aphids during its life.

Several insecticides have come onto the market in the last ten years which provide good levels of aphid control. The newer products tend to be safer materials that are used at very low rates. The somewhat bad news is that these new products cost a lot more than some of the older pesticides that have been around longer.

Some of the materials we have tested at the University of Maryland Extension with good success in aphid control include Aria (flonicamid) and Endeavor (pymetrozine). Both block the stylet of the aphid, resulting in starvation of the insect. In our trials it took 5 -10 days before you really see the impact of the application but it is very effective for aphid control. It’s difficult for insects to develop resistance to a material that does something physical like blocking the stylet. Another new material we trialed in 2009 is Kontos (spirotetramat). A systemic insecticide, Kontos was very effective controlling green peach and melon aphid for 20 -30 days. It is also good on spider mites.

To help determine insecticide efficacy, several aphid-infested plants can be marked with flags or flagging tape, and an estimate of the aphid number on each may be recorded. Several days after an insecticide application, the number of live aphids may be recorded. It is important to examine plants carefully and frequently to determine whether additional applications are required.

New Publication

This winter, Karen Rane (pathologist), Andrew Ristvey (horticulturist), Joyce Latimer (horticulturist) and myself (Stanton) have collaborated on a new extension publication for greenhouse production. We hope “Total Plant Management for Greenhouse and Cut Flower Management – with Emphasis on IPM and Nutrient and Water Management” will be in print and online later this spring.

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]