HORTSCIENCE 30(5):997–999. 1995. plant growth and yield responses of muskmel- ons to imidacloprid in the presence and ab- Imidacloprid Does Not Enhance sence of whiteflies. Growth and Yield of Muskmelon in the Materials and Methods Greenhouse studies. All plants used in the Absence of Whitefly greenhouse tests were direct-seeded ‘Topmark’ muskmelons in a 3 soil : 3 perlite : 1 peat J.C. Palumbo and C.A. Sanchez mixture in 1.5-liter pots. Each pot contained 500 g of soil mixture and was planted with four University of Arizona, Yuma Valley Agricultural Center, 6425 West 8th Street, to five seeds. Seedlings were grown during Yuma, AZ 85364 Mar. and Apr. 1994 in a glasshouse under natural light with adequate water and nutrients Additional index words. Bemisia tabaci, Bemisia argentifolii, Cucumis melo, relative growth for maximum growth. Upon emergence, seed- rate, net assimilation rate ling plants were thinned to one per pot. Pots Abstract. Imidacloprid is a new, chloronicotinyl insecticide currently being used to control were then placed in wooden-frame exclusion × × sweetpotato whitefly [Bemisia tabaci Genn, also known as silverleaf whitefly (Bemisia cages (1.7 m width 1.2 m long 0.6 m high) argentifolii Bellows and Perring)]. Large growth and yield increases of muskmelon screened with fine organdy cloth to exclude (Cucumis melo L.) following the use of imidacloprid have caused some to speculate that this whitefly adults and other insects. The cages ± compound may enhance growth and yield above that expected from insect control alone. were maintained in the glasshouse at 28 4C. Greenhouse and field studies were conducted to evaluate the growth and yield response of Whitefly adults used in these studies were melons to imidacloprid in the presence and absence of whitefly pressure. In greenhouse of mixed age and from a population originally cage studies, sweetpotato whiteflies developed very high densities of nymphs and eclosed collected from melons at the Yuma Valley pupal cases on plants not treated with imidacloprid, and significant increases in vegetative Agricultural Center. The colony was main- plant growth were inversely proportional to whitefly densities. Positive plant growth tained on muskmelon plants in an outdoor responses were absent when plants were treated with imidacloprid and insects were insectary under ambient conditions. Samples excluded. Results from a field study showed similar whitefly control and yield responses of our whiteflies that were maintained in the to imidacloprid and bifenthrin + endosulfan applications. Hence, we conclude that growth insectary were b-strain sweetpotato whitefly and yield response to imidacloprid is associated with control of whiteflies and the or silverleaf whitefly. subsequent prevention of damage, rather than a compensatory physiological promotion The effects of imidacloprid and whitefly of plant growth processes. Chemical names used: 1-[(6-chloro-3-pyridinyl)methyl]-4,5- feeding on seedling growth and vigor was dihydro-N-nitro-1-H-imidazol-2-amine (imidacloprid); [2 methyl(1,1´-biphenyl)- investigated in two separate cage studies. In 3yl)methyl 3-2-chloro-3,3,3-trifluoro-1-propenyl]-2,2-dimethylcyclopropane carboxylate the first trial, imidacloprid was applied to 60 (bifenthrin); 6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3- seeded pots by drenching the soil of each pot benzodiaxathiepin 3-oxide (endosulfan). with 0.00, 0.02, 0.04, or 0.08 g a.i. in 18 ml of water per pot (Mullins, 1993). Immediately The sweetpotato whitefly, also known as whitefly populations in muskmelons (Palumbo following application, each pot was hand- silverleaf whitefly (Bellows et al., 1994), has et al., 1994). Imidacloprid was first used com- watered to provide adequate moisture and recently become a serious economic pest of mercially in the United States in 1993 for initiate germination of the seed. The experi- muskmelons (Perring et al., 1993). Damage by sweetpotato whitefly on lettuce (Lactuca sativa ments were conducted as a randomized com- whitefly species to most crops includes a re- L.) and muskmelons in Arizona and Califor- plete-block design with five replications. Each duction in plant growth caused by the removal nia. Because yields improved in most fields exclusion cage served as a replication. Within of plant assimilates from the phloem during following imidacloprid applications (Palumbo, each cage, three pots of each rate were ran- feeding (Buntin et al., 1993) and the excretion 1994b), observers speculated that imidacloprid domly assigned a position within the cage for of honeydew, which promotes the growth of may have a plant growth regulator (PGR) a total of 12 plants per treatment. In the second sooty mold on harvestable plant parts (Byrne effect on plants that can increase yield above trial, imidacloprid was applied at 0.00, 0.005, and Miller, 1990). Fruit quality and yield of that expected from insect control alone. 0.010, or 0.020 g a.i., as indicated above. muskmelon can decrease when immature Plant growth and yield enhancement has When plants reached the one true-leaf stage, whitefly population densities are not properly been attributed to some insecticides by several ≈1200 whitefly adults (100/plant) were col- controlled (Palumbo, 1994a). investigators (Parrot et al., 1985; Scott et al., lected from the insectary and released into Imidacloprid (Admire 2F, Miles, Kansas 1985). However, in many cases, more rigor- each cage to allow for oviposition and nymphal City, Kan.) is a new chloronicotinyl insecti- ous evaluations have shown that plant growth development. Adults and nymphs were al- cide that is toxic to many insect pests (Elbert et and yield responses are associated entirely lowed to feed and develop for 35 days and then al., 1990). It has good systemic insecticidal with a reduction in insect pressure. For ex- removed from the cages for assessment of activity in the plant and performs best when ample, early reports that N´-(4-chloro-o-tolyl)- whitefly densities and plant growth. Whitefly applied to the soil to be absorbed by roots N,N-dimethylformamidine (chlordimeform) densities were estimated by counting the num- (Mullins, 1993). This chemical has demon- enhanced cotton (Gossypium hirsutum L.) ber of eggs, nymphs, and pupal cases from a strated exceptional control of sweetpotato growth (Campbell et al., 1979) were later random 2.5-cm2 area of the six oldest leaves on shown to be largely associated with a reduc- each plant. Plant growth responses were based tion in insect damage (Cathey and Bailey, on the dry weights (milligrams) of leaves, 1987; Youngman et al., 1990). Because petioles, and stems of each plant that had been imidacloprid is applied to crops as a prophy- dried at 70C for 48 h in a forced draft. Leaf area Received for publication 10 Nov. 1994. Accepted lactic control measure at planting, the poten- (square centimeters) was measured with a LI- for publication 17 Apr. 1995. A portion of this tial for the overuse of this compound is highly 3100 leaf-area meter (LI-COR, Lincoln, Neb.). research was supported by a U.S. Dept. of Agricul- possible if growers suspect a plant growth– ture–Agricultural Research Service Cooperative The dry weight and leaf area measurements Agreement no. 58-5344-3-456. The cost of publish- insecticide interaction. However, if no stimu- were used to calculate mean relative growth ing this paper was defrayed in part by the payment latory effects on plant growth can be associ- rate (R) and mean net assimilation rate (E) of page charges. Under postal regulations, this paper ated with imidacloprid, growers are likely to according to Hunt (1978). therefore must be hereby marked advertisement use the chemical more judiciously. Therefore, Plant growth response to imidacloprid in solely to indicate this fact. the objective of this study was to evaluate the the absence of whitefly was evaluated in ex- HORTSCIENCE, VOL. 30(5), AUGUST 1995 997 CROP PRODUCTION clusion cage studies using the same design as were found on treated plants (Table 1), the ing by sweetpotato whitefly nymphs reduced noted above. Imidacloprid was applied to 60 study was repeated using sequentially lower leaf chlorophyll content of tomato seeded pots by drenching the soil of each pot insecticide rates (Table 2). Although a few (Lycopersicum esculentum Mill.) leaflets and with 0.00, 0.02, 0.04, or 0.08 g a.i. in 18 ml of nymphs were located on plants receiving 0.005 adversely affected rates of leaf photosynthesis water per pot. The experiment was conducted or 0.01 g a.i./pot (Table 2), nymph densities and CO2 assimilation. three times under similar greenhouse condi- were >70-fold higher in the control plants. Imidicloprid induced no positive plant tions. Plant growth responses were assessed as This density : rate response is inversely pro- growth responses when insects were excluded explained above. portional to the insecticidal activity of from muskmelon plants. Analysis of three We calculated mean whitefly densities per imidacloprid on whiteflies (Oetting and Ander- separate cage studies showed no significant (P square centimeter per leaf, R, and E. Data from son, 1990). Pupal cases existed only on the > 0.05) increase in plant growth relative to all greenhouse studies were analyzed by sub- nontreated plants, indicating that whiteflies applied rates of imidacloprid (data not shown). jecting mean whitefly densities, mean dry had completed immature development and Pooled analysis showed that mean plant dry weights, leaf areas, E, and R to trend analysis adults had emerged during the experimental weight (range 0.80 to 0.84 g) and mean total using General Linear Models procedure of period. We could not quantify the feeding leaf area (200 to 230 cm2) per plant were SAS (SAS Institute, 1988). effects of adults on the plants because of the similar for each rate of imidacloprid and the Field study.