The Clone Wars: Aphids!
There is much talk about how we as a society must stopcloning at all costs. Just think about what would happen if humans were tobecome like aphids. Sexual reproduction would no longer be necessary, and allfemales would be born pregnant.
Well all right, that’s going a little far, but that’s whathappens to an aphid in the greenhouse. For example, the green peach aphid isknown to reproduce sexually outside in nature, but in the controlledenvironment of the greenhouse, they reproduce asexually. They are all females,and yes, they are all born pregnant. When aphids give birth, they give birth tolive young rather than eggs, and they are more or less born pregnant. Theycannot give birth until they become an adult, but the young inside of thenewborn aphid are already developing–their reproductive capacity is amazing.It’s exponential. Needless to say, pest populations can appear to developovernight. In fact, scientists of old used the sudden appearance of aphids onplants as proof for spontaneous generation — the theory that living organismscan originate in nonliving matter.
According to the literature about aphids, every plant on theplanet can be fed upon by one or more aphid species. However, when oneconsiders that there are more than 4,000 species of aphids described worldwide,there are relatively few that are ornamental pests.
Aphid Biology
Aphids are small, pear-shaped, soft-bodied insects varyingin color from light green to dark brown, and they are commonly found on theundersides of leaves or collecting around the terminal buds. Aphids possesscornicles that extend from the end of the abdomen, a key morphological featurethat separates them from other insects. Cornicles are tube-like structures, andaphids use them to excrete alarm pheromones, warning other aphids when they areattacked. The cornicles and the cauda (the tail more or less) are used foraphid species identification.
Aphids have piercing, sucking mouthparts and feed from plantphloem tissues. They process the proteins from the fluid, leaving the majorityof the carbohydrates (sugars) for excretion. Those carbohydrates are excretedas a sweet, sticky juice, commonly referred to as honeydew. If left unattended,the honeydew will allow the growth of sooty mold, turning the plant surfaceblack and unsightly. The presence of ants is a good indicator of an aphidinfestation because they readily feed on honeydew. If you can follow an anttrail in the greenhouse, you can usually identify an infestation of aphids orsome other related family member such as whiteflies or mealybugs.
Aphid development is dependent on species and temperature,but in general, they grow to maturity in 5-7 days. Adult aphids usually do nothave wings, but winged forms are more common in large populations. It isimportant to control aphids before they disperse throughout the greenhouse.
Early detection
Plant damage from aphids includes the following: 1) simplepresence of the pest and cast skins; 2) honeydew and sootymold; 3) transmissionof phytopathogens; and 4) a general decline in health of the plant as exhibitedby yellowing, leaf distortion and stunting. If any of these symptoms areevident, the populations are already quite high and are going to be difficultto control.
Certain species of aphids have demonstrated very high levelsof resistance to pesticides. Therefore, it is critical to locate and eliminatesmall populations of aphids so that frequent applications of chemicals are notnecessary. Monitoring for aphids may require a professional scout or awell-informed team of growers and workers. It is most important to visuallyinspect the plants and growing areas, and it may require a hand lens for visualinspection. Experience and common sense tells us that the more intimate arelationship the grower or worker has with the plants, the easier it is toidentify a potential problem before it gets out of hand.
Sticky traps can be used for monitoring adult alate (winged)aphids though the presence of winged adults usually means the population isalready large. There is some debate in the scientific community over whetheralatae are formed because of population size or other biochemical processes;regardless, you should be aware that they are lurking in your crop. Traps canwarn of aphid presence, hot spots and pest migration and can be used to give arelative measure of the effectiveness of pesticide treatments. You should useone trap per 1,000 sq.ft. Place the traps a few inches above the plant canopyand move them up as the plants grow. Check the traps weekly, and keep a goodrecord of trap counts. These are the most important records you can keep. Youmay be able to identify trends in pest pressure on different cultivars or immigrationfrom a specific location.
Exclusion
Make the growing area as pest-free as possible beforeplanting. This includes removing crop debris, old plants and weeds andsterilizing the soil. Obviously, you should also start with pest-free stockplants. There are other things, however, that can be employed to excludeunwanted greenhouse pests. Aphids can be excluded with exclusion screening,screening vents and entrances, and restricting entry into growing areas.Growers and other workers should avoid Á wearing colors that attractaphids because aphids can be hitchhikers. Weeds or even well-cared-forlandscape outside the greenhouse will harbor aphids, which can enter thegreenhouse through vents or open sides.
Control
Aphid reproduction occurs so rapidly that reapplication ofpesticides for control, if necessary, should occur in 5-7 days. In addition,pesticide class should be rotated every 2-3 weeks.
The chloronicotinyls Marathon and Flagship continue to bevery good products against aphids. Two old standbys, the carbamate Mesurol andthe organophosphate Pinpoint, also prove to be very effective against aphids.Of particular note are the soil-applied treatments in Figure 5, page 17. Soilapplications successfully make their way into the phloem where they areaccessible to piercing, sucking insects like aphids. Some newer products showgood promise for aphid control for the future. BSN-2060 from Bayer, Flonicamidfrom FMC, Pedistal from Uniroyal and V-10112 from Valent cause significantmortality of aphids compared to the control. Some resurgence is noted in acouple of these products at lower rates, which means they may require repeatedapplications for success. In theory, that means they may have a less likelychance of causing pesticide resistance due to the lack of persistence of thechemical.
So, yes, you as a grower may be caught up in a clone war, awar against that persistent foe the aphid. However, never fear, they are stillsusceptible to most registered pesticides, and there will be a number of backupproducts available soon.
Note: Figure 3, page 17, lists the pesticides used in recenttrials at the University of California-Riverside, and Figures 3-5, page 17, aresummaries of recent trials. All rates in the figures are per 100 gallons unlessnoted. Some of the pesticides in the figures are registered for use on theintended target, and others are experimental because it is always good to knowthere is hope in future products. We do not always use labeled rates in ourtrials because these trials are for experimental purposes only. However, ourknowledge about the products and their capabilities grows from every trial.Labels constantly change. Therefore, it is always the pesticide applicator’sresponsibility, by law, to read and follow all current label directions for thespecific pesticide being used. No endorsement is intended for productsmentioned, nor is criticism meant for products not mentioned.
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