Second Place Student Essay

Therioaphis maculata—Spotted Alfalfa

Sarah Haymaker

HERIOAPHIS MACULATA, or the spotted alfalfa aphid, is adults, except their coloration is lighter (Bieberdorf and Tindigenous to the western Palaearctic region, or Central Bryan, 1956). Asia and Southern Europe (Sunnucks et al., 1997). It has Spotted alfalfa have antennae with four to six been recorded from the Middle East and several countries segments. The antennae are held along the lateral part of the bordering the Mediterranean Sea. In 1953, the spotted alfal- upper body surface when at rest, and are extended when the fa aphid was reported in New Mexico, and in 1954 it was aphid is in motion. Winged and non-winged forms occur found in much of the Southwestern United States. Within throughout the year. The wings are transparent, with the three years, it was reported in most states in the continental hindwings smaller than the forewings (Bieberdorf and United States except those bordering Canada. The spotted Bryan, 1956). alfalfa aphid has now been reported in all states and in Spotted alfalfa aphids, like all Homoptera, have piercing- Canada. Most recently, the spotted alfalfa aphid has migrat- sucking mouthparts. The beak consists of two pairs of ed to Australia and New Zealand. This widespread distribu- stylets formed by the maxillae and the mandibles, all tion of the spotted alfalfa aphid is due to its ability to travel sheathed in the labium. The beak appears to rise from on air currents (Cameron et al., 1983). between the thoracic legs, thus distinguishing the Homoptera from the Hamiptera (Stehr, 1990). Other Habitat appendages include the cornicles and a cauda, or tail. The The spotted alfalfa aphid is a monophagous , mean- cornicles are found on the ventral side of the aphid on seg- ing it only has one host plant species during its life cycle. ments five and six. The cornicles secrete honeydew, or The spotted alfalfa aphid feeds almost exclusively on alfal- aphid excrement (Arnett, 1985). fa, although it can survive and reproduce on nineteen legume species (Sunnucks et al., 1997; Cameron et al., Reproduction 1983). In Oklahoma, the spotted alfalfa aphid spends the The spotted alfalfa aphid has a remarkable life cycle that winter at the crown of the alfalfa plant. Here, both the non- is characterized by a series of parthenogenetic generations winged adults and immature stages are found on or around followed by a biparental generation. This life cycle provides the crown, but their feeding and movement is limited by an easy and rapid way to increase the numbers that are need- lack of warmth and moisture (Bieberdorf and Bryan, 1956). ed to survive lack of food, unfavorable weather conditions, When colonizing an alfalfa plant or field, spotted alfalfa and numerous predators (Arnett, 1985). aphids infest the lower leaves of older plants first. The Spotted alfalfa aphids are paurometabolous, having egg, aphids prefer feeding in the lower portions of the canopy nymph, and adult stages. Reproduction is usually accom- and have little tendency to move upward (Berberet et al., plished by parthenogenetic, viviparous females, meaning 1991). As the number of spotted alfalfa aphids increase, females that produce live young without fertilization from their feeding results in necrosis and loss of leaf blades, forc- males. In late summer, both males and females are produced ing the aphids to migrate to leaves in higher portions of the to accomplish fertilization and production of eggs to over- canopy (Cameron et al., 1983). winter. In warm climates, eggs may not ever be produced (Bieberdorf and Bryan, 1956). Physical Description In the spring, the eggs hatch into nymphs. The nymphs The spotted alfalfa aphid was first described as develop into wingless females. The wingless females, called Chaetophorus maculatus (Buckram) in India in 1899 stem mothers or fundatrix, reproduce parthenogenetically (Cameron et al., 1983). Six hiatuses are now recognized in (Price, 1997). These nymphs undergo four molts and a short the western United States (Nielson et al., 1970). When maturation period of one to four weeks, depending on the mature, the spotted alfalfa aphid is soft-bodied and oval temperature (Bieberdorf and Bryan, 1956). After this peri- shaped. It is approximately 0.15 cm in length, with a yellow od, the progeny begin to deposit their own offspring. Many body and eight rows of brown spots. The spots are parallel reproducing aphids will then undergo one to two additional to the long axis of the body and form parallel bands arranged molts. There are a total of four immature instars. segmentally. Each spot has a spine, which is wider at the tip Offspring will continue to be reproduced parthenogenet- than at the base. Immatures appear virtually the same as ically as long as conditions such as weather and food supply permit, producing up to thirty generations per year. When food supply lessens or summer ends, females develop Department of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, wings. Winged females migrate to new host plants. As win- OK 74078. ([email protected]) Received 7 June 1999. ter approaches, sexual forms are produced and eggs are Published in J. Nat. Resour. Life Sci. Educ. 28:84–86 (1999). formed. These are the stages that overwinter to begin the http://www.JNRLSE.org cycle again in the spring (Arnett, 1985).

84 • J. Nat. Resour. Life Sci. Educ., Vol. 28, 1999 Environment kill dormant alfalfa plants or plants not growing due to drought. Other problems associated with the feeding of spot- Spotted alfalfa aphids are found all over the world, but ted alfalfa aphids are stand-thinning, poor seed set, and they survive and reproduce the best in semi-arid environ- shortened stand life (Davis et al., 1972). ments. Temperatures as low as -12°C restrict aphid activi- It has been theorized that the feeding of the spotted alfal- ties and cause death of those not protected by the alfalfa fa aphid causes damage other than direct fluid loss from the crowns or debris (Bieberdorf and Bryan, 1956). Tempera- tures of 20 to 25°C are optimum for reproduction (Cameron plants. There is the possibility that viruses are transmitted et al., 1983). Optimum relative humidity for the spotted through the aphid saliva that is exchanged with the plant. alfalfa aphid is 25 to 30% and higher humidities are detri- Another theory is that the spotted alfalfa aphid is capable of mental to the aphids (Angalet, 1970). producing toxins that are injected into alfalfa plants, increas- Predators are another environmental factor that are ing the rate of necrosis (Davis et al., 1972). important in survival of the spotted alfalfa aphid. Many The tremendous amount of honeydew production associ- species prey heavily on the aphid including larval and adult ated with the spotted alfalfa aphid also causes additional ladybird beetles, lacewings, damsel bugs, Syrphid flies, geo- problems in alfalfa production. Populations of forty aphids corids, braconids, chalcids, cynipids, and some fungi (Price, per stem will begin to cause stickiness in alfalfa plants eight 1997). to eighteen inches tall (Dickson et al., 1955). Machine har- vest becomes very difficult when honeydew concentrations Special Attributes increase. For example, harvest machinery must be frequent- ly washed and balers cannot produce quality bales. In addi- Spotted alfalfa aphids have many special characteristics tion to mechanical harvesting difficulties, mold growth is that enhance survival in an ever changing environment. One rapid on the honeydew covered leaves and stems. The black of these is biotype formation and sexual forms that allow powdery mold that thrives on honeydew reduces the live- survival in colder climates. Another characteristic is the stock acceptance of the alfalfa hay (Davis et al., 1972). nature of aphids to be territorial. To ensure that the young will have a sufficient food supply, female aphids will have a Control kicking and pushing match. This determines which aphid will get the 3 mm of leaf area required to feed the new born The first and most important control method is the devel- nymphs (Price, 1997). opment of resistant varieties. Several alfalfa varieties have Spotted alfalfa aphids occur in winged and non-winged been developed that are tolerant to apparent toxins and feed- forms throughout the year. Winged aphids occur in greatest ing damage (Davis et al., 1972). Other control methods numbers in late spring and early summer. These aphids can include the introduction and establishment of parasites of fly at 1.9 to 2.4 km h-1 to colonize new areas (Price, 1997). the spotted alfalfa aphid. Strip harvesting, or leaving uncut Elimination of these wings is a protective measure to pre- strips on a given harvest date to conserve and enhance nat- vent from being blown away (Arnett, 1985). ural enemies is another beneficial control method. The final control method is chemical control, or use of Feeding insecticides. An insecticide should be chosen based on its ability to control the spotted alfalfa aphid as well as preserve Aphids pierce plants with stylets to suck phloem from the the natural enemies. Timing of insecticide application will vascular bundles. At this time, the spotted alfalfa aphid may inject a toxin, but studies are inconclusive. Since aphids do also help preserve the natural enemies. Common insecti- not destroy the photosynthetic machinery of the plant, they cides that are used to control the spotted alfalfa aphid are can extract large amounts of energy per square meter of veg- Furadan, Lorsban, Malathion, Parathion, and Pounce (OSU etation (Price, 1997). Of this energy consumed by the Extension, 1996). aphids, 5% goes to production, but 90% is excreted as hon- For all methods of control, the development of useful eydew. The limiting factor in the aphid’s food is the level of economic thresholds and implementation of sampling pro- amino nitrogen. Therefore, the spotted alfalfa aphid has cedures for aphids and natural enemies should be considered adapted a feeding system that allows it to take in an exces- (Pedigo, 1996). The economic injury level for spotted alfal- sive quantity of food to obtain required nutrients. fa aphids is approximately twenty aphids per stem for ten inch alfalfa plants and forty aphids per stem for twenty inch Damage alfalfa plants (OSU Extension, 1996). These injury levels should be monitored closely to prevent aphid damage. When Several types of damage can be seen on the alfalfa plant controlling the spotted alfalfa aphid, all previous methods of due to spotted alfalfa aphid feeding. The first and most obvi- control should be taken into consideration and sampling for ous damage is necrosis of leaf tissues and leaf loss. During this pest is imperative. warm weather periods, the aphids feed on the lower leaves of the plant. Following necrosis of the leaves, the leaves begin to drop off the plant. The aphids then proceed up the References stem, feeding on leaves higher in the canopy. Eventually, the plant will exhibit a bare stem and only a tuft of leaves is left Angalet, G.W. 1970. Population, parasites, and damage of the spotted alfal- at the top of the plant (Dickson et al., 1955). Spotted alfalfa fa aphid in New Jersey, Delaware, and the eastern shore of Maryland. J. Econ. Entomol. 63:313–315. aphid feeding also slows the growth of the alfalfa plant and Arnett, R.H., Jr. 1985. American insects: A handbook of the insects of may halt the growth of new shoots completely. Feeding can America North of Mexico. Van Nostrand Reinhold Co., New York.

J. Nat. Resour. Life Sci. Educ., Vol. 28, 1999 • 85 Berberet, R.C., R.W. McNew, J.W. Pillwith, and J.L. Caddel. 1991. Within- Pedigo, L.P. 1996. Entomology and pest management. Prentice Hall, plant patterns of maculata on resistant, tolerant, and sus- Englewood Cliffs, NJ. ceptible alfalfa plants. Environ. Entomol. 20:551–555. Price, P.W. 1997. Insect ecology. John Wiley & Sons, New York. Bieberdorf, G.A., and D.E. Bryan. 1956. Research on the spotted alfalfa Nielson, M.W., H. Dan, M.H. Schonhorst, W.F. Lehman, and V.L. Marbel. aphid. Oklahoma Agric. Exp. Stn. Bull. B-469. 1970. Biotypes of the spotted alfalfa aphid in western United States. J. Cameron, P.J., R.G. Sunde, and G.P. Walker. 1983. Discovery, identifica- Econ. Entomol. 63:1822–1828. Ohio State University. 1996. OSU extension agents’ handbook of insect, tion, and host plant interaction of spotted alfalfa aphid (: plant disease, and weed control. p. 141. ) in New Zealand. N. Z. J. Agric. Res. 26:511–517. Stehr, F.W. 1990. Immature insects. Kendall/Hunt Publishing Co., Davis, D.W., M.P. Nichols, and E.J. Armbrust. 1972. Bibliography of the Dubuque, IA. spotted alfalfa aphid. The literature of associated with alfal- Sunnucks, P., F. Driver, W.V. Bryan, M. Carver, D.F. Hales, and W.M. fa. p. 3–4. Milne. 1997. Biological and genetic characterization of morphological- Dickson, R.C., E.F. Laird, Jr., and G.R. Pesho. 1955. The spotted alfalfa ly similar (Hempitera: Aphididae) with different aphid. J. Agric. Sci. Calif. Agric. Exp. Stn. 24:93–117. host utilization. Bull. Entomol. Res. p. 425–436.

86 • J. Nat. Resour. Life Sci. Educ., Vol. 28, 1999