Month: December 2020

One of the real delights of the holiday season is adding festive plants to the home décor. As we have seen in previous episodes of Bug of the Week, this often means that some six and eight-legged creatures including mantises, adelgids, and spiders sometimes accompany a Christmas tree when it arrives indoors. These plant-related holiday surprises are not limited to Christmas trees, oh no. This week, let’s visit another fascinating but dastardly insect that sometimes makes its presence known on one of my favorite holiday plants, poinsettia. Decorating the home with poinsettias is a holiday tradition that likely originated in Mexico, where poinsettias add beauty to holiday nativity crèches. According to legend, in the town of Cuernavaca a young girl had no flowers to adorn the nativity at her church. Instead, she collected a weed growing by the roadside. An angel transformed the weed into the beautiful poinsettia, and ever since, poinsettias have been used as a seasonal decoration. Poinsettias are as much of a holiday tradition as mistletoe, holly, and an evergreen tree in many homes.  

As I browse displays of poinsettias that have sprung up in every hardware and grocery store, I’m on the lookout not just for holiday decorations, but for holiday whiteflies. Whiteflies are small sucking insects and relatives of more commonly known sap-suckers such as beech and woolly alder aphids we met in recent previous episodes. To spot a whitefly-infested poinsettia, first look at the color of the plant’s leaves – not the red, yellow, or speckled bracts comprising the blossom. The leaves on most varieties should be a clear deep green with no evidence of yellow patches or streaks. Leaves with yellow patches that are undersized, twisted, puckered, or curled can be the ghost of holiday problems yet to come. Whiteflies are usually found on the underside of leaves where they insert tiny beaks into the plant’s vascular system to extract sugar-rich sap from a vascular element called phloem. Plant sap contains nutrients needed for their growth and development. Large populations of whiteflies can cause leaves to drop prematurely. As whiteflies feed, they excrete a sugary waste product called honeydew. This sticky liquid can become a substrate for the growth of an ugly fungus known as sooty mold.  

Whiteflies have four stages in their lives, egg, nymph, pupa, and adult. An adult female whitefly lays eggs on the undersurface of a leaf. After several days, eggs hatch and mobile nymphs, called crawlers, move about the leaf surface until they find a suitable place to feed. Nymphs hunker down, shed their skin, and become stationary for the remainder of their youth. After several molts, a pupa forms, and from this pupa emerges the adult whitefly. Shed pupal skins often festoon the undersides of leaves. Adult whiteflies look like tiny white moths. They too have sucking mouthparts and suck the sap of poinsettias. If disturbed, they flutter from the leaf surface. The nymphs and pupae are usually yellowish or whitish and translucent. Red eyespots can be seen on the pupa shortly before the adult whitefly emerges.  

With wings yet to unfold, an adult whitefly emerges from its pupal case. A bit later with wings now fully expanded, polishing up the thorax and antennae is the next order of business to get ready for the holidays.

For the folks that grow poinsettias, whiteflies can be a very serious problem. If populations become too great, entire crops are lost. The most common whiteflies that come home with poinsettias are the greenhouse whitefly, Trialeurodes vaporariorum, and the silverleaf whitefly depicted in this Bug of the Week. A strain of silverleaf whitefly, the dastardly “Q biotype”, has been found in several states in the US. This strain is resistant to many of the pesticides formerly used to control whiteflies and causes headaches not only for the greenhouse industry but also for growers of some of our most important agricultural crops including tomato, peppers, squash, cucumber, beans, eggplant, watermelon, cabbage, potato, peanut, soybean, and cotton. Fortunately, entomologists are finding new ways to control whiteflies using tiny wasps that attack them, predatory beetles that eat them, and pathogens that give them lethal infections.  

If this Bug of the Week sounds a bit like the Nightmare before Christmas, try not to panic. Don’t let a few whiteflies on your poinsettia spoil the holiday. A few whiteflies will not ransack your poinsettia between now and the New Year and, hey, many poinsettias will join other decomposing vegetation in the compost well before Valentine’s Day. However, if whiteflies are numerous and your poinsettia looks whipped, and you had planned to keep your poinsettia going with the other house plants, it may be best to toss it out and replace it with one not bearing tiny six-legged gifts. Many fine, pest-free poinsettias can still be had in retail markets large and small. So, Happy Holidays to poinsettias, whiteflies and all!

 Acknowledgements

 The University of Florida Extension publication “Sweetpotato Whitefly B Biotype, Bemisia tabaci (Gennadius) (Insecta: Hemiptera: Aleyrodidae)”, by Heather J. McAuslane and Hugh A. Smith, was consulted in preparation for this episode of Bug of the Week. Carol Of The Bells by Audionautix is licensed under a Creative Commons Attribution 4.0 license: https://creativecommons.org/licenses/by/4.0/ Artist: http://audionautix.com/

Bug of the Week wishes everyone a joyous, happy, and safe holiday season.

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With winter fast upon us and most deciduous trees and shrubs already naked, my sweet autumn clematis, Clematis terniflora, still shines like a beacon of green as it engulfs a lamppost in the front yard. After rewarding me with fragrant blossoms for much of the fall, this non-native invader is hosting an entire food web of alien insects. Let’s start with the herbivores, a flock of peach-clematis aphids. During spring and summer, through Thanksgiving, and, in this era of climate change perhaps all winter, my clematis will be home to peach-clematis aphids. Several species of clematis and peaches serve as food for this now cosmopolitan vagabond. Beneath each leaf, scores to hundreds of aphids suck nutritious phloem sap. Over the past several months and in years past, the aphid horde is so abundant that clematis leaves drip with honeydew, which in turn serves as a substrate for the growth of sooty mold. This type of non-pathogenic fungus, akin to one produced by boogie woogie aphids and woolly alder aphids we met in previous episodes, blackens leaves and may interfere with photosynthesis.

The saga of the peach-clematis aphids began earlier this year in the spring when winged aphids migrated from a tree in the peach clan (Prunus) to the clematis vine clinging to my lamppost. During the warm months of summer and fall, surviving females begin reproducing without assistance of males. This form of reproduction, called parthenogenesis, produces only females, thus enabling aphid populations to increase rapidly. As I examined my colony of aphids recently, I noticed several winged adults mixed with the parthenogenetic females. In cold regions like Maryland, when temperatures turn chilly, male and female winged adults are produced and leave the clematis, returning to Prunus to mate and lay eggs that will spend the winter on the bark of the tree. Sexual reproduction in the fall produces eggs, the overwintering stage of the aphids. In spring, the eggs hatch and the complex life cycle of the aphid resumes.

Another fact of life contributing to explosive population growth of aphids is their ability to skip the usual insect-like business of laying eggs. Many species of aphids dispense with the egg stage and, like humans, give live birth to their babes. This blessed event takes only a few minutes but appears to be fraught with significant drama. Birthing aphids do lots of posturing and pushing. Fortunately, aphids have sucking mouthparts and loud vocalizations such as those accompanying human births are conspicuously absent. To further accelerate the process of filling the world with their kind, female aphids carry embryos of their grandchildren within their bodies even before they are born (i.e. their daughters are born already pregnant). This further compresses the generation time for aphids and is part of the reason aphid populations rapidly grow from a few to thousands.

With Thanksgiving in the rear view mirror and the rest of Holiday Season just around the corner, peach-clematis aphids are still living it up on sweet autumn clematis. On a leaf blade a mother is surrounded by her daughters, and on a petiole aphids suck sap from phloem. Just for fun, watch the live birth of an aphid (five times actual speed). Note that aphid births are breech. At the end of the clip, is that older sister coming to see how mom and little sister are doing? Dance of the Sugar Plum Fairy by Kevin MacLeod is licensed under a Creative Commons Attribution 4.0 license, https://incompetech.com/music/royalty-free/index.html?isrc=USUAN1100270. Artist: http://incompetech.com/

Just as my clematis serves as a feast for aphids, so too will the aphids and their honeydew serve as dinner for guests higher up the food chain. Roaming around my clematis, multicolored Asian lady beetle larvae snack on aphids. Their alligator-like larvae patrol leaves and stems searching for tasty aphids. Without much stealth or finesse, larvae capture aphids in their jaws and proceeded to munch their hapless prey. Small aphids disappear in just a minute or two, but large, plump aphids required several minutes to eat. A single larva of the multicolored Asian lady beetle may devour 1,200 aphids during the course of development. Adults of this species can also kill hundreds of aphids during their lifetime. This capacity to eat so many aphids makes the multicolored Asian lady beetle one of the most effective biological control agents in our gardens.

In this microcosmic food web, clematis is food for the peach-clematis aphid, and aphids will become food for both adult and larval lady beetles. A yellowjacket laps up carbohydrate rich honeydew produced by the aphids. This rich source of carbohydrates may help yellowjackets fatten up to survive a wicked winter. Dance of the Sugar Plum Fairy by Kevin MacLeod is licensed under a Creative Commons Attribution 4.0 license: https://creativecommons.org/licenses/by/4.0/, Source: http://incompetech.com/music/royalty-free/index.html?isrc=USUAN1100270, Artist: http://incompetech.com/

And during this season of Holiday feasts, what banquet would be complete without dessert? With gobs of sweet honeydew on leaves, dozens of hungry yellowjackets recently visited the clematis to lap up a carbohydrate rich meal to fatten up for their winter rest. So, in this festive season, while we devour turkey and savor pumpkin pie, reflect on the happy feasts underway on greenery like an autumn clematis where aphids dine and serve dinner to other creatures higher up the food web.

Acknowledgements

Bug of the Week thanks Roger Blackman and his amazing website, Aphids on the World’s Plants: An online identification and information guide at http://www.aphidsonworldsplants.info/ which served as a reference for this episode. The fact-filled leaflet “Multicolored Asian Lady Beetle” by Janet Knodel, E. Richard Hoebeke, and Carolyn Klass, Dept. of Entomology, Cornell University provided great information on the ladybeetle. Many thanks to Dr. Gary Miller of USDA’s Systematic Entomology Laboratory for his help in identifying the tiny star of this episode.

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One of the most interesting and charming woody plants in our landscapes is the boxwood. This evergreen shrub has been domesticated by gardeners since the time of the Roman Empire. Pliny wrote about the uses of boxwoods as a garden hedge and as a source of wood for the construction of musical instruments. Boxwoods are featured in the Holiday season in North America and Europe where their glossy, deep green leaves grace wreaths, centerpieces, and garlands in homes. This week on a trip to my favorite market, I passed a boxwood hedge that looked anything but charming. The usually deep green leaves of the boxwood were mottled yellow and orange and disfigured with bumps and blisters. The plants appeared to have a bad case of pox. What misfortune had befallen these noble shrubs? After plucking a few leaves from the embattled boxwoods, and carefully removing the lower surface of a leaf, I discovered several tiny, squirming, yellow maggots just beneath the surface. These were the larvae of the diabolical boxwood leafminer.

The boxwood’s blues began last spring in early May when adult boxwood leafminers emerged from leaves, exiting their leafy nursery. On the way out, the adult flies left behind the shed skin of their former pupal case. The papery skins, called exuviae, protrude from the bottom surface of the leaf for many days before dropping from the plant. Adult boxwood leafminers are delicate orange flies closely resembling mosquitoes. After mating, the female fly seeks the undersurface of a young boxwood leaf. Using a small drill-like structure at the tip of her abdomen, she punches a tiny hole in the surface and lays eggs in the soft tissue beneath. From these eggs tiny yellow boxwood leafminer larvae hatch. During summer and autumn, as the larvae grow, boxwood leaves produce small circles of cells surrounding developing larvae. These cells serve as a source of food for the maggots and produce abnormal swellings of the leaf, called galls. Development slows during the chilly months of winter and early spring, but by April, with the return of warmer temperatures, larvae grow rapidly as the boxwood ramps-up its activity. To escape the protected confines of its mine, the leafminer has a clever trick. Just before pupating, the maggot moves to the lower surface of the leaf and carefully removes almost all of the leaf tissue until only a thin layer is left. This layer forms a window that will be used by the pupa as an escape hatch. With a successful exit strategy in place, the larva pupates and in a few short weeks the pupa pushes through the window, enabling the adult fly to escape.

Peeling back the lower epidermis of a boxwood leaf reveals boxwood leafminer maggots nestled within, ready for a long winter’s nap. Fast forward to next spring, when larvae have completed development and pupated. As they emerge from leaves, shed pupal cases mark the exit sites of adult midges. With jazzy twisting motions, the rockin’ female drills into a tender new leaf to deposit eggs.

Managing leafminers

If you are troubled by boxwood leafminers, then a good, non-insecticidal way to thwart this uninvited guest is to plant varieties of boxwoods resistant to attack by this fiend. Years ago, we discovered that a variety of boxwood from the highlands of Macedonia was highly resistant to the boxwood leafminer. This variety is called Buxus sempervirens ‘Vardar Valley’. You can find it in several nurseries in the DMV. Here’s hoping your boxwoods don’t suffer holiday blues courtesy of the boxwood leafminer.

References

A wealth of information on boxwoods can be found in Lynn Batdorf’s wonderful “Boxwood Handbook”, used as a reference for this story. Information on resistant boxwood varieties came from the article entitled “Integrated approaches for managing the boxwood leafminer, Monarthropalpus flavus” by M.J. Raupp, I.H. Mars, and G. d’Eustachio, the abstract of which can be found at this link:  https://www.actahort.org/members/showpdf?booknrarnr=630_6

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