|Yard & Garden Line News
Volume 7 Number 8 June 1, 2005
How Do Those Bugs Eat?
Charlie Rohwer, Graduate Research Assistant, Horticultural Science
Help, my roses are covered in fine webbing and look sick! My ficus has little bumpy things proliferating on the stems! My nasturtiums are being overwhelmed by tiny green bugs that pop when I squish them! You identify your respective problems as spider mites, scale, and aphids, you take appropriate measures to control or eliminate your unwanted invertebrates, and then you forget about it. You are unfortunately well aware of what they eat. But you really want to probe more deeply, you want to understand your enemy: you want to know how they eat. So here it is, an introduction to feeding mechanisms for the most feared invertebrate pests of home and garden.
Aphids on red salvia
All Photos: Charlie Rohwer
Aphids on blue salvia.
Fungus gnat larva
The basic mechanisms of feeding for herbivorous invertebrates are chewing, phloem feeding, and cell feeding. Some insects, like flies, 'sponge' their food, and butterflies and moths siphon their food, but these generally aren't pests associated with our gardens or houseplants.
Chewers: How about starting off with the worst of the worst of garden pests, the rabbits and deer. Yep, they chew alright, and they're great at it. But they're a separate topic. Invertebrates include the 'spineless' insects and spiders that live on our plants. So, the chewers we're talking about include caterpillars, beetles, fungus gnats, and grasshoppers/locusts. These pests all eat by chewing up plant tissues with their powerful mandibles (jaws). They crush up their food and shove it in to be digested. This mechanism is the most analogous to the way we eat-our jaws and teeth rip, tear, and crush the food to make it fit down our throats and aid digestion.
Caterpillars are larvae of lepidopterans that will grow up to become moths and butterflies. They can neatly snip tissues at the ground, in the case of cutworms, shape odd holes in the leaves, like cabbage loopers and hornworms, bore through a fruit, like codling moth larvae, or carve weird pale lines through the leaves, like leafminers.
Beetles can cause damage in both larval and adult forms in some cases. The white grubs that wreak havoc on the roots of your grass grow up to be the beautiful, yet destructive Japanese beetles that skeletonize the leaves of your favorite plants. Some beetles and their larvae damage trees by boring through them. The elm bark beetle transmits Dutch elm disease in this way as it busies itself chewing through your elms. The mouth parts of beetles point forward from the head.
Fungus gnats are an annoyance for houseplants. The adults can sometimes be found flitting around an overwatered plant, but the adults don't do much damage. They're a bigger annoyance for greenhouse growers who propagate cuttings, because the larvae greedily chew on the young, growing roots. They can prevent roots from forming, eventually killing the cuttings.
Grasshoppers/locusts, classified into the same order as the katydid, cricket, praying mantis, and cockroach, are famous for devouring large fields of crops. Grasshoppers have no larval stage like caterpillars, beetles, and flies, they just start of as tiny grasshoppers with powerful teeth, and get bigger from there. Unlike beetles, the mouth parts of grasshoppers point downward.
Phloem feeders: Imagine sticking a straw into a maple tree, then lapping at the sap through the straw. That is similar to how sucking insects feed, like aphids and leafhoppers. They poke their feeding tube (stylet) directly into the phloem tubes in soft tissues that the plant uses to shuttle liquid food around. Then they can suck up the nutrient-rich fluids that the plant has made. Taxonomists separate aphids and leafhoppers into separate orders. Leafhoppers are accurately called "bugs" (technically, only insects in the order 'Hemiptera' are bugs). Whiteflies, scale, aphids, and mealybug are phloem-feeding insects more closely related to each other. They can attack houseplants with vigor, and some plants in the garden are especially susceptible to certain species.
Cell feeders: Mites and thrips are considered cell content feeders. Like the phloem feeders, they don't actually chew off and swallow any plant tissues. Instead, they poke or rasp a hole in individual cells on the leaf, then they either suck up the juice from inside the cell or suck it up as it seeps from the cell. Think of a juice box. The straw is analogous to the stylet the insect pokes into the cell ('juice box') to feed, although thrips and mites have to try a little harder to get their 'straw' into the cell than we do. Thrips have unique structures to get at the cell contents. They have a left mandible used to scrape into the cells, but they have no right mandible; thrips are lop-sided! Mites are not actually insects-they're more closely related to spiders. Most mites don't eat plants, but those that do can be a hassle.
If all of this talk about how these pests eat makes you feel a little creepy, maybe you fell like you should go sterilize your garden and all of your houseplants, not to worry. You'll never be able to keep it sterile. Just follow good growing practices and remember, you're not at war with nature. Many insects are actually pretty helpful in the garden. And if your ficus is covered in scale, sucking away at the phloem, that's a great opportunity (excuse) to replace it with something you never had the space for before!
Lewis, T. 1973. Thrips. Academic Press, London.
Pedigo, L. 1999. Entomology and Pest Management. Prentice-Hall,Upper Saddle River, New Jersey.
Join us June 2, when MPR's Midmorning radio program is at the University of Minnesota Landscape Arboretum
9-10 AM Carroll Henderson, DNR Non-game Wildlife Expert
10-11 AM Deb Brown
Come see live a live radio broadcast. Stay for the Wild About Birds exhibition.
Admission $7 non-members. Members and children 15 & under get in for free.
The X Files: Hosta Virus X
Janna Beckerman, Extension Plant Pathologist
Hostas are the most popular perennial grown in the Midwest and northeastern portions of the US. They are mainly grown for their beautiful foliage, shade tolerance, ability to take neglect and abuse, and their disease resistance. And hosta lovers will agree with every word I just said-except the disease resistance part.
Classic symptoms of Hosta Virus X.|
Like all plants, hostas suffer from a myriad of disease problems. Until recently, viral diseases in hosta were paid scant attention. To make matters worse, the most "famous" hosta virus that infects the cultivar 'Sea Sprite' turned out to not be a virus at all. So you can imagine the surprise in the hosta world about Hosta Virus X (HVX), which is turning up wherever hostas seem to be grown, and being found in hosta that aren't even symptomatic!
For this reason. HVX is prevalent, and underdiagnosed. Problems arise because plants can be asymptomatic when you (or the nursery) purchase them, but develop symptoms weeks, months or years later. Large numbers of HVX infected plants are currently being sold in nurseries, despite the efforts of regulators and nursery owners. To date, thousands of hostas have been destroyed once they were identified.
A definitive diagnosis requires laboratory testing, but there are some tell-tale symptoms you should be on the look out for. Avoid hostas that have irregular color feathering, regardless of the variety (It's difficult to keep the sheer number of hosta cultivars straight, but try, or at least, know what you buy and make sure it is what it says!) Some commonly infected cultivars include 'Gold Standard,' 'Striptease,' and 'Sum and Substance'). The problem is reported to be so severe in 'Gold Standard' that named forms have developed based upon the symptomology of the infection, and H. 'Breakdance' is actually named for the color breaking pattern due to virus infection-just like tulipmania in Holland! Symptoms are reported to be more pronounced in gold and gold-centered plants, like 'Golden Tiara' and 'Striptease.' Symptoms are difficult to describe, but resemble golden feathering spreading out of the leaf veins. Fortunately, excellent photos on different cultivars can be seen at http://www.hostalibrary.org
How it Spreads
HVX is transmitted primarily through vegetative propagation (dividing plants). Contact with sap from the infected to sap of a healthy plant can result in infecting the healthy plant. Simple acts of dividing hostas, scape removal, and removing leaves can potentially spread this virus. Like all plant viruses, HVX requires a living host, and cannot survive if the host hosta dies, so new plants may be planted where a virused plant was removed, as long as there are no still-living roots remaining. It's important to note that a few cultivars are considered to be resistant or even immune to HVX (See Table).
HVX is a serious disease-however, host range does seem limited to hosta, and it can be managed by exclusion (keeping out of ones nursery or garden) and sanitation (removing infected plants, good propagation practices). I only wish regulators and nursery owners would actively test and destroy hosta and other plants infected with tobacco rattle virus, or foliar nematode, both of which have a much larger host range, and the potential to cause widespread and more severe damage.
Table 1. Reaction of Popular Hosta to HVX (from Blanchette and Lockhart, 2004*).
||H. sieboldiana 'Elegans'
For a complete list and more information, see: Hosta Virus X: A three year study. 2004. B. Blanchette and B. Lockhart. Hosta Journal 31 (2): 19-23.
For more information on HVX on-line, see: http://www.inthecountrygardenandgifts.com/articles/hosta_virus_x.shtml
Striped Cucumber Beetles
Jeffrey Hahn, Assist. Extension Entomologist
Keep your eyes open for the striped cucumber beetle, Acalymma vittatum. This insect is a periodic pest in home gardens of cucurbits, i.e. cucumbers, squash, melons, and pumpkins. The striped cucumber beetle generally occurs in low numbers from year to year but occasionally is very abundant and damaging to plants.
Striped cucumber beetle|
You can recognize a striped cucumber beetle by its 1/5 inch long oblong body, its yellowish green wing covers, and its three longitudinal black stripes. It also has a black head and antennae as well as an orangish prothorax (the first area behind the head).
There are a couple of similar looking insects you might also see in your garden. The spotted cucumber beetle is similar in size, shape, and color and also likes to eat cucurbits but is much less commonly encountered. Instead of stripes, this beetle has 12 black spots on its wing covers. The western corn rootworm is also yellow green with three black stripes but has a darker colored prothorax and a yellow-green abdomen underneath (a striped cucumber beetle has a black abdomen underneath). Although you may find this beetle feeding on cucurbits blossoms, it is not a pest on squash family plants.
A striped cucumber beetle overwinters in protected sites on the ground in wooded areas and fence lines adjacent to cucurbit fields. It is unclear whether it actually survives winters around the Twin Cities and areas to the north. Limited studies show that they are not tolerant of the cold and usually die. It is possible that in cases of mild winters some may survive. These areas are repopulated each spring by striped cucumber beetles flying in from the south. It does appear that striped cucumber beetles do overwinter successfully in southern Minnesota.
When the temperature warms to an average of 65ºF, generally late May or early June, these beetles emerge and feed on the blossoms of early flowering plants, such as dandelions, apples, and hawthorn, until their host crops are available. Once cucurbit crops are growing, adult striped cucumber beetles fly to gardens, feed on the leaves, and lay eggs at the base of plants. This feeding is the most damaging to cucurbit plants.
Plant Disease Clinic
Eggs hatch in several weeks and whitish larvae feed on plant roots and underground parts of stems. Despite this feeding, the larvae are not usually considered to be pests. The larvae pupate in the soil and emerge later in the summer as adults. It takes about 40 to 60 days for this insect to go from an egg to an adult. Adults return to cucurbit plants and feed on the foliage later in the summer. When populations are high, they can also chew on stems and fruit. There is typically one generation per year.
Striped cucumber beetles are also injurious because they can infect cucurbits with bacterial wilt. These beetles harbor the bacteria in their bodies and infect feeding wounds with contaminated feces. All cucurbits can become infected, however, cucumbers and melons are most susceptible and likely to die from bacterial wilt while squash and pumpkins are generally tolerant to it. Infected plants wilt and leaves can discolor before eventually dying. Once a plant is infected with bacterial wilt it can not be saved.
You can test a plant for bacterial wilt by cutting it with a knife and then squeezing the ends. Infected plants should ooze a milky white bacterial substance. You can also hold the two cut ends together and then slowly pull them apart. If it has bacterial wilt, you should observe a string of bacterial ooze.
Don't confuse bacterial wilt with other cucurbit problems. Two insect pests, squash bugs and squash vine borers can inflict damage that can cause leaves to wilt. Watch for heavy numbers of squash bugs or orangish frass exuding from stems (a symptom of squash borer damage). Squash family plants can also exhibit wilting leaves when suffering from drought. For more information on bacterial wilt, click here, http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/p260cucurbit-bactblt.html.
When managing striped cucumber beetles, plants are most susceptible to damage when plants are in their first to the third true (or normal) leaf stages, i.e. the leaves that emerge after the first seed leaves. Watch your plants regularly for striped cucumber beetles during this time.
If you find at least 25% defoliation during this time, you should treat your plants to protect them from these beetles. Use a garden insecticide labeled for cucurbit crops. Common examples include permethrin, esfenvalerate, or carbaryl. It isn't important to treat striped cucumber beetles you see later in the summer as they do little or no damage to plants.
It is very difficult to protect cucurbit plants from bacterial wilt. If you have a history of this disease in your garden, you can try to protect your cucurbits by erecting a floating row cover or similar barrier during early to mid-June to keep the striped cucumber beetles away from your plants. Be sure to remove the barrier when your cucurbits start to flower.
Jeffrey Hahn, Assist. Extension Entomologist
Mosquito season is upon us. And once again we contemplate how to best protect ourselves from their annoying bites as well as West Nile virus and other diseases they can transmit. Last year in Minnesota, we experienced 34 cases of West Nile virus, including two that resulted in human death.
Photo: Spectrum Brands, Inc.
Off contains DEET
Oil of Lemon Eucalyptus
There are several steps you can take to minimize mosquitoes in your yard, such as draining containers that can hold water, keeping tall, weedy grass cut, turning off outside lights or using less attractive lights like sodium lights. You can also minimize your exposure to mosquitoes by avoiding times when mosquito activity is the highest, i.e. dawn and dusk and wearing protective clothes, especially long-sleeved shirts and long pants .
Probably the best measure you can take to protect yourself from mosquito bites is using a repellent. The best overall repellent has traditionally been DEET also known as N,N-diethyl-m-toluamide. DEET has been available to the public since 1956 and has been consistently effective. DEET has a excellent track record of safety with relatively few incidences of adverse reaction to its use. Since 1960, there have been fewer than 50 reports of serious toxic effects in the medical literature out of an estimated 8 billion applications. And most of those cases resolved themselves without any secondary effects. Still some people do not like DEET because they may be sensitive to it while others don't like its smell or the feel of it on their skin.
When using DEET, or any repellent, always read the label so you can use the product correctly. When applying DEET, use just enough repellent to lightly cover exposed skin or clothing. Avoid over-applying DEET; heavy application doesn't increase protection. Don't apply it under clothing or over cuts, wounds, or irritated skin. Wash treated areas with soap and water after returning indoors. Also wash treated clothing before wearing them again.
It is generally accepted by the American Academy of Pediatrics and other experts that you can apply 30% DEET or less to infants over 2 months and children. When applying DEET to children, adults should first apply the repellent to their own hand and then rub it on the skin of the child. Avoid applying DEET into their eyes and mouth. Do not apply DEET more often once a day.
Adults should consider how long they want to be protected from mosquitoes when deciding which DEET concentration to use. Research has shown that 23.8% DEET protected people from mosquito bites up to five hours, 20% DEET up to four hours, 6.65% DEET up to two hours, and 4.75% DEET protected up to 90 minutes. However, there appears to be a limit to how much protection increasing concentrations of DEET can provide. A product containing 100% DEET can repel mosquitoes for no more than 10 hours. There is evidence that suggests that there may not be much difference between concentrations of 35% and 100%.
In a recent Centers of Disease Control and Prevention (CDC) article, they reaffirmed that one of the most effective and long-lasting repellents registered by the Environmental Protection Agency (EPA) is still DEET. However, they went on to say that there are now two additional options for repellents in the war against mosquitoes. The CDC hopes that with more choices, people will be more willing to use repellents to protect themselves from mosquito transmitted diseases, such as West Nile virus.
One of these options is Picaridin (KBR 3023). Picaridin has long been used as a mosquito repellent in Europe, Australia, Latin America, and Asia. It's available in the U.S. for the first time this year. Recent research determined that this repellent was comparable to lower concentrations of DEET in effectiveness. Picaridin is also reputed to be less irritating to skin. You can find this active ingredient as a 7% concentrate in Cutter Advanced Insect Repellent. This product is marketed as lacking any chemical odor and sticky feel while also being fragrance free.
Another repellent endorsed by the CDC is oil of lemon eucalyptus (p-menthane 3,8-diol or PMD), a plant-based repellent. It is comparable to products containing low concentrations of DEET. It is sold under a variety of product names, such as Repel Lemon Eucalyptus.
Bite Blocker containing 2% soybean oil is also mentioned by the CDC as an option. Research has shown that this repellent can offer protect for about 90 minutes or about the same protection as a very low concentration (4.75%) of DEET.
The CDC also suggests using insecticides containing 0.5% permethrin for use on clothing, shoes, netting, tents, and other camping gear. They protect the user by killing mosquitoes that come in contact with it. Unlike repellents, don't apply permethrin to your skin. Examples of this insecticide include Permanone and Sawyer Tick Repellent. Permethrin is also the active ingredient in the Ex Officio Buzz Off line of clothing. You can increase your protection from mosquitoes by applying a repellent to your skin as well as using permethrin on your clothing.
Get the low down on this month's insect pests at
June Garden Calendar
Compiled from conversations with Bob Mugaas, Nancy Rose, Patrick Weicherding Regional Extension Educators
Trees and shrubs
'Lemon Lights' azaleas.
Now is the time to prune confiders--to control size and correct shape. Candles are the only part you can prune on pines. Do not cut into last year's growth.
Now's a good time to fertilize trees and shrubs if they show symptoms of chlorosis, poor performance or a soil test indicates a need.
ware that broadleaf weed control products that contain dicamba will injure trees and shrubs. Even 2,4-D can damage foliage. Boxelder and green ash are quick to show 2,4-D herbicide injury symptoms. Here's another reason fall is the best time to kill weeds--the trees are going dormant and less susceptible to injury.
Pre-emergence herbicides containing trifluralin, such as Preen, will interfere with feeder root development of trees and shrubs. Stressed plants are susceptible to secondary invaders.
Continue to look for wilt diseases--Dutch elm, oak wilt and verticilium.
Don't prune oaks for at least another month.
Silver maples and elms are producing bumper corps of seeds. Consequently, they are slower to leaf out and may appear sparse until their leaves expand.
Ash anthracnose has been reported. It is typified by the trees dropping new leaves. For information see: http://www.extension.umn.edu/yardandgarden/ygbriefs/p413ashanthrac.html
Spruce sawflies are out. If you see worms eating needles of spruce, pine or tamarack, click on
Keep those trees and shrubs you planted this spring mulched and watered.
Prune lilacs shortly after they're done blooming.
If your area has received a great deal of rain so far this spring, an early June application of fertilizer, providing ½ lb. of nitrogen per 1,000 sq ft. would be beneficial. Nitrogen has been removed from the soil due to leaching and used up due to all the growth.
If you put down a pre-emergence crabgrass preventer application in later April, now would be a good time for a second application. Focus your efforts on areas where crabgrass has been a problem in the past. You don't need to treat your entire lawn.
It's still ok to spray broadleaf weeds. Avoid applying on hot days (see label), windy days and of the soil is really dry resultiug in drought-stressed plants.
Now is the time almost all lawn grasses flower and set seed. Unfortunately, the seeds that you can see are immature and will not re-seed a thin lawn. Seeds reach maturity some time in July. Flowering shoots will die, making the lanws appear thinner.
Continue to mow regularly and a bit more frequently especially with such favorable growth conditions. It's better to mow frequently to avoid removing too much of the plant at once.
Fruits, Flowers and Veggies
Rhubarb is in bloom. Cut off the flower head as it's faster to get new rhubarb plants from divisions rather than seeds.
Now is the time to plant warm season veggie transplants. Plant on a cloud day or shade them to reduce transplant shiock.
Use paper towel or toilet paper rolls, ads cards from magazines or other stiff paper to protect stems of tomatoes, pepper and eggplants from cutworms. This is the height of the cuteworm season.
Annuals should be planted. Still time to seed cosmos, marigolds, sunflowers
People who want to spray apples for apple scab and cedar apple rust should be spraying now.
It's also a critical time to spray for plum cuculio. For details see: the Home Fruit Spray Guide at
People who are willing to bag apples so they don't have to spray for anything should do it now. See:
I looked back at the June 1, 2004 issue. I reported then that we'd had 11 days straight of rain. This year we've also off to a soggy start with about 2/3 of the last 30 days having some rainfall.
The tulips above are 'Georgette" that started as yellow amd turned red as they matured.
Nancy Rose had promised us something for the June 15th issue. In later issues: Patrick Weicherding has agreed to write about what happens to trees when lightning strikes. Julie Weisenhorn will continue the landscaping project story in a future issue. And Bob Mugaas will be co-authoring an article on the changing face of home herbicides.
Please feel free to cut and paste any of the articles for use in your own newsletters. All we ask is that you give our authors credit.
Back issues Yard & Garden Line News are on the Yard & Garden Line home page at www.extension.umn.edu/yardandgarden/.
Deb Brown answers gardening questions on Minnesota Public Radio's (MPR) "Midmorning" program on the first Thursday of every month at 10 a.m. The program is broadcast on KNOW 91.1 FM, and available state-wide on the MPR news radio stations. (Scroll down for map.)
The Yard & Garden Clinic closed December 12, 2003 due to budget cuts. Questions from the general public will no longer be accepted. Samples will not be accepted.
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