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Cotton Aphid Emerges As Major Pest in SJV Cotton

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Cotton Aphid Emerges As Major Pest in SJV Cotton he cotton aphid (Aphis gossypii TGlover) has escalated from an oc- casional pest to a severe pest during the last 10 to 15 years in the SJV. This has occurred in a series of steps. From Cotton aphid emerges about 1986 to 1991, the highest aphid densities were generally observed on major pest in SJV cotton cotton before squaring and again after boll opening. In 1992, high aphid den- sities developed midseason (July and Larry D. Godfrey u Jay A. Rosenheirn o Peter B. Goodell August). This trend continued and reached a maximum in 1995 and 1997, During the 199Os, the cotton aphid cotton aphid reproductive rates when the cotton aphid was recorded (Aphis gossypii Glover) escalated and can create conditions favor- as one of the most economically im- from an occasional pest to an an- able to aphid outbreaks. The new portant arthropod pests of SJV cotton, nual, severe pest of San Joaquin practice of promoting vigorous replacing the traditionally important Valley (SJV) cotton. Although the growth may have created an opti- pests such as spider mites (Tetrunychus cotton aphid is not a new insect in mal host plant environment for spp.) and lygus bugs (Lyphesperus). the SJV, the change in the bio- cotton aphid reproduction and In 1997, cotton aphids in California nomics of this pest dictated that survival. In addition, the shift to cotton caused an estimated $34 million new research efforts be directed using pyrethroids to control other in crop loss and another $38 million in control costs. Overall, insect and mite toward its biology, damage arthropod pests has enhanced the pests in California cotton in 1997 thresholds and management. The buildup of cotton aphid popula- caused an estimated $66 million and 1980s saw major shifts in agro- tions. The severe aphid outbreak $167 million in crop loss and control nomic practices. For example, the during the 1995 cotton growing costs, respectively. During 1998 and introduction of a plant growth season served as a focal point to 1999, cotton aphid outbreaks were regulator allowed growers to pro- bring the industry together to dis- moderate and spotty; however, un- mote vigorous plant growth, cuss the status of cotton inte- usual weather patterns (El Niiio and rather than limiting growth and grated pest management and to La Niiia) were an overriding factor in yield potential through the prac- plan future directions. Results of SJV cotton production. tice of water and nitrogen stress. this effort include increased aware- The cotton aphid is not a new insect Recent field research has shown ness, greater understanding of cot- in the SJV; Swift in the 1950s reported that applying high rates of nitro- ton aphid biology and improved on the biology of this pest (Swift 1958). gen to cotton plants increases pest management decisions. Cotton aphid outbreaks, especially in 26 CALIFORNIA AGRICULTURE, VOLUME 54, NUMBER 6 the cotton production area in eastern white, powdery wax on their bodies. tion of these early-season aphid infes- Tulare County, have occurred sporadi- Dark cotton aphids (dark green to tations (because natural enemies at- cally. However, changes in the bion- black) were found to develop more tack them), suggested that early-sea- omics and importance of this pest re- rapidly, give birth to more offspring, son aphids should be viewed as quired that updated research be and grow larger than light (light yel- "non-pests." In a limited region of the conducted on its biology, damage low) aphids. Dark cotton aphids were SJV, early-season aphid populations thresholds and management. Reasons also more tolerant of pesticides. There- may persist, but in most of the valley a for the "emergence" of cotton aphid as fore, the dark aphids were implicated conservative treatment approach, such a key pest may be linked to changes in in the population explosions in the as not treating with insecticides and cotton production practices and in in- field. In a laboratory study, the factors waiting for natural enemies to deci- secticide use patterns. associated with the production of dark mate the populations, is warranted. The 1980s saw major shifts in agro- morph aphids were identified as During the cotton squaring and nomic practices due to improved vari- cooler temperatures, shorter day boll-filling period, cotton aphids com- eties and an emphasis on yield. The in- lengths and nutrient-rich host plants pete directly with the cotton plant's re- troduction of more determinate (Rosenheim et al. 1994). However, the productive structures for energy. The fruiting varieties resulted in a more influence of these factors and the role phloem-feeding aphids compete for compact growing season. During the of the aphid phenotypes in cotton the photosynthates along with the de- same time, mepiquat chloride was in- aphid population dynamics in the veloping cotton bolls. Severe aphid troduced to control vegetative growth field have not been clearly elucidated. feeding can result in smaller or fewer and improve fruit retention. The intro- Knowledge of cotton aphid biology bolls per plant, reducing yield. Re- duction of this plant growth regulator during the winter (off-season period search by Godfrey and coworkers allowed growers to promote vigorous for cotton) is important in designing (1997) supported a treatment thresh- plant growth, rather than limiting alternative management plans. Can old of 50 to 100 aphids per leaf (fifth their growth and yield potential management methods that would main-stem node leaf) for 7 to 10 days. through the practice of water and ni- positively influence aphid manage- Following boll-opening, cotton trogen stress. Recent field research has ment in cotton be implemented, such aphids have the potential to deposit shown that water stress to cotton as releases of natural enemies or the honeydew (excrement) on the exposed plants depresses cotton aphid repro- destruction of weed hosts? Studies cotton lint. This threat is very serious ductive rates and can cause heavy concentrating on the winter and early for the cotton industry because sooty mortality of reproductive adults, and spring periods have shown that the mold grows on honeydew, staining that aphid reproduction is enhanced sexual phase of the cotton aphid sea- the fiber, and the stickiness of the hon- on nitrogen-rich plants. This new crop sonal life history occurs on pomegran- eydew gums up harvesters and pro- management practice - of developing ates. Alate males, oviparae (female cessing equipment in gins and textile vigorously growing plants during the aphids that mate and produce eggs), mills. A treatment threshold of 10 to 15 early- and midseason - may have cre- eggs and fundatrices (asexual female aphids per leaf is supported by ated an optimal host plant environ- aphids that develop from eggs) were Rosenheim's research and others. ment for cotton aphid reproduction all found on this plant species. Winter and survival. In addition, the shift to annual weeds and citrus were also Biocontrol strategies new chemical classes, specifically syn- identified as hosts for overwintering Several studies have been con- thetic pyrethroid materials, to control asexual cotton aphids, but at much ducted over the last 10 years on vari- other cotton arthropod pests has en- lower densities than the sexual forms ous aspects of natural control of cotton hanced the buildup of cotton aphid found on pomegranates. aphids in the SJV. Biological control is populations. an important natural control measure Economic thresholds for several arthropod pests of SJV cot- Cotton aphid biology The development of treatment ton. Rosenheim et al. (1997) showed With the initial outbreaks of aphids thresholds was a primary emphasis that predators and parasitoids effec- in cotton in 1986 and 1987, there was during the past 10 years of cotton tively reduced aphid populations on some uncertainty regarding the spe- aphid research in California, as no ex- pre-squaring-stage cotton. However, cies involved. Rosenheim et al. (1994) periments had previously been con- during the mid- and late season, bio- identified the pest as Aphis gossypii ducted to establish the impact of cot- logical control of the cotton aphid is and clarified that the cotton aphid ex- ton aphids on cotton plants grown generally poor; at this time, the com- ists as several phenotypes. The color under California conditions. munity of natural enemies found on variation extended from light yellow Rosenheim et al. (1997) showed that cotton is dominated by generalist to green to dark green to black. cotton can fully compensate for aphid predators, primarily lacewings and Nymphal aphids that are developing infestations during the pre-squaring predatory bugs such as big-eyed bugs, into winged or alate adults (alatiform stage. The ability of early-season cot- minute pirate bugs and assassin bugs nymphs) may also be amber colored, ton to compensate fully for aphid (Cisneros and Rosenheim 1997). Al- and some alatiform nymphs secrete feeding, and the generally short dura- though lacewings can, when tested CALIFORNIA AGRICULTURE, NOVEMBER-DECEMBER 2000 27 The cotton aphid exists as several phenotypes. The color variation extended from light yellow to green to dark green to black. Photos and digital composite by Jack Kelly Clark. alone, suppress aphid populations ef- fied resistance to several organophos- al. 1999). Balancing the needs of the fectively, lacewings are themselves phate insecticides and to endosulfan in cotton crop for maximum production suppressed by the predatory bugs, the mid- to late 1980s in the SJV. In the with the level that mitigates aphid which eat the lacewing larvae. Because early 1990s, cotton aphids in California population buildup remains an unre- the predatory bugs eat relatively few developed resistance to bifenthrin. In solved challenge. aphids, overall suppression of aphid 1996, Grafton-Cardwell found that Nitrogen level in the plant also in- populations by the predatory insect 83%, 71% and 42% of the aphid popu- fluences aphid susceptibly to insecti- guild is weak.
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