Pesticides May Reduce Lettuce Yield Frank V
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High-value crops such as strawberries, tain classes of insecticides on lettuce photo- usually clearly visible. Insecticides applied at broccoli, and iceberg lettuce often receive synthesis, transpiration, and productivity. normal rates and under the right environ- “preventive” or “insurance” pesticide treat- mental conditions may subtly damage a plant ments, which may result in weekly scheduled Insecticides but remain unobserved, because symptoms applications of insecticides. Many times such Insecticides in the various “classes,” such are not visible. treatments are unwarranted economically as chlorinated hydrocarbons (DDT, endrin, During the last few years, plant physiolo- and may reduce yields by detrimental effects and methoxychlor), organophosphates gists at University of California, Riverside, on the plants. Decreases in strawberry yields (guthion, parathion, and methyl-parathion), have developed the dual isotope porometer, due to preventive insecticide treatments in the carbamates (malathion and methomyl), and which provides accurate, simultaneous mea- absence of economically significant pest synthetic pyrethroids (fenvalerate and per- surements of a plant’s photosynthesis and populations have been reported. Research methrin), differ in their effects on plants. Ad- transpiration rates in the field. Entomologists supported by the California Iceberg Lettuce ditionally, the rates, number, and timing of have used the instrument to measure effects Research Advisory Board indicated that head applications may alter a compound’s effect of mite feeding on almond, cotton, avocado, lettuce plots kept insect pest-free were gener- upon the plant, for either a short time or sev- and strawberry foliage. In this study, we used ally not the highest yielding plots. Further eral days. the porometer to measure the effects of cer- analysis of the data indicated that a high Many compounds are phytotoxic when ap- tain insecticides on the lettuce plant’s number of insecticide applications before let- plied at high rates or under certain environ- photosynthesis and transpiration rates. tuce head formation may reduce lettuce mental conditions, such as extreme heat or In the 1979 investigation, Climax variety yields. We report here on the effects of cer- moisture. Resulting damage to the plant is lettuce was winter-grown at the U.C. South Pesticides may reduce lettuce yield Frank V. Sances 0 Nick C. Toscano Marshall W. Johnson 0 Larry F. LaPre’ Author Frank Sances demonstrates use of porometer in lettuce field. Dual isotope porometer developed by plant physiologists at U.C., Riverside, simultaneously Mean weights of lettuce heads harvested measures effects of insecticides on lettuce photosynthesis and transpiration. from plots treated with one of three insec- ticides once and twice weekly. 4 CALIFORNIA AGRICULTURE, NOVEMBER-DECEMBER 1981 Coast Field Station (SCFS), Santa ha.Four week after treatment, overall photosynthesis productivity. The higher value of transpira- compounds, representing the previously had decreased to 20 percent of the normal tion occurred on insecticide-free plants on mentioned insecticidal classes were applied to rate. Similar trends were observed with the both sample dates. Photosynthesis was also the lettuce at the following rates of active in- other insecticides investigated. Methyl- higher in the untreated check, suggesting that gredient per acre: methomyl (Lannate, Nu- parathion induced an initial 17 percent reduc- all insecticide treatments adversely influenced drin) at 0.9 and 1.8 pounds, methoxychlor tion in transpiration and 10 percent reduction both stomatal opening and overall photosyn- (Marlate) at 2 and 4 pounds, methyl- in photosynthesis, which decreased to 27 and thesis rates and that the effect remained for at parathion (Red Top Methyl Parathion) at 1 18 percent reductions, respectively, after one least one week. Reductions of lettuce head and 2 pounds, and permethrin (Pounce, Am- week. weight and diameter in plants treated weekly bush) at 0.2 and 0.4 pound by a C02-charged Permethrin had the most detrimental ef- with methyl-parathion indicate the cumula- row crop sprayer. An untreated check was in- fects on transpiration on both sample dates. tive effect on yield that results from pesticide cluded. Applications were also made at night Photosynthesis rates of permethrin-treated inhibition of physiological processes. to detect whether the insecticides penetrated plants were reduced most significantly on the Since no differences were observed be- the plant through the stomates (which are first sample date. By the second date, high tween the photosynthesis and transpiration closed at night). The plant’s photosynthesis variation among samples due to leaf age rates of plants treated in daylight or darkness, and transpiration rates were measured on the obscured statistical significanceof the insecti- it may be assumed that insecticide uptake by fist and eighth days after treatment to cides’ influence on photosynthesis. the plant is independent of stomatal opening. observe both acute and chronic effects of the No significant differences were detected compounds. when chemicals were applied in darkness or Conclusions In 1980, Great Lakes and Mesa lettuce var- sunlight, nor did the two rates of each chemi- Methyl-parathion (organophosphate) and ieties were planted at the Citrus Research Sta- cal applied alter responses measured. permethrin (pyrethroid) had the greatest ef- tion (CRS), Riverside, and effects of three Data collected at the CRS indicated that fects on photosynthesis and transpiration compounds on lettuce head productivity were neither variety nor number of insecticide ap- rates. Methyl-parathion significantly re- studied. Experimental plots were divided into plications per week appeared to reduce let- duced lettuce head yields. Studies on other two groups, which received treatments either tuce head weight and diameter significantly. crops have shown that parathion reduces once or twice weekly. The following amounts However, lettuce sprayed weekly during the photosynthesis, possibly by inhibiting certain of active ingredient per acre were applied at entire growing season weighed significantly processes in the chloroplast where photosyn- each application: methomyl at 0.9 pound, less (22.1 ounces) than that not treated during thesis takes place. methyl-parathion at 1 pound, and fenvaler- the growth period from thinning to head for- Unwarranted insecticide treatments with ate (Pydrin) at 0.2 pound. In each group, mation (23.7 ounces). methyl-parathion in the absence of econom- half of the plots were sprayed during the Lettuce plants sprayed weekly with both ically significant insect populations may whole season from germination to harvest, weekly rates of methyl-parathion produced reduce yields by reducing photosynthesis and and the other half were treated from ger- heads that weighed significantly less than transpiration. However, although methoxy- mination to thinning and from head forma- those in other treatments (see graph). Methyl- chlor least adversely affected lettuce plants, tion to harvest. parathion-treated heads were significantly and permethrin and methyl-parathion caused smaller in diameter (about 7 percent) than the greatest reductions in physiological pro- Plant responses those in the check and plots treated with both cesses, when relative effectiveness in regulat- All insecticides examined at SCFS adversely weekly rates of methomyl and the low weekly ing a pest population is considered, the latter affected photosynthesis and transpiration rate of fenvalerate. compounds are more desirable. rates but differed in their relative effect and As more data are obtained on the effects of severity of injury (see table). On both sample Plant physiology pesticides, systematic screening of new com- dates, untreated plants had highest rates. Transpiration is directly related to stomatal pounds for their impact on photosynthesis Methoxychlor-treated plants did not have opening, which is essential for leaf intake of and consequently on yield may become essen- significantly different photosynthesis and carbon dioxide for photosynthesis. Photo- tial. Our results indicate that the dual isotope transpiration rates than untreated plants. synthesis is a more general measurement of porometer has great potential in instantane- Transpiration and photosynthesis declined carbon dioxide intake and fixation (sugar ously measuring subtle changes in plant pho- 9 and 8 percent, respectively, 24 hours after production). Both processes are interrelated tosynthesis and transpiration rates in the field plants were treated with methomyl. One and are directly related to plant growth and and in quantifying recovery over time. Devel- opment of a “pesticide threshold” that would indicate the maximum number of ap- plications at recommended rates on desig- Effectof insecticide sprays on photosynthesis and transpiration rates of lettuce leaves nated crops may thus be desirable. One day post treatment’ Eight days post treatment’ Insecticide Photosynthesist Transpiration* Photosynthesist Transpiration# Untreated check 18.93 a 2.54 a 17.90 a 1.84 a Frank K Sances, former Post-graduate Research Met hoxychlor 18.06 ab 2.41 a 16.15 a 1.64 ab Assistant, University of California, Riverside, is Methornyl 17.40 abc 2.31 ab 15.15 a 1.49 b now head of Pacific Agricultural Laboratories, Methyl-parathion 16.97 bc 2.12 b 14.72 a 1.47 b San Diego; Nick C. Toscano is Extension Entomol- Perrnet hrin 16.25 c 2.11 b 14.72 a 1.34 b ogist, U.C., Riverside; Marshall W. Johnson is Post-graduate Research Assistant, C., River- ‘Mean separations in vertical columns by Duncan s new multiple range test (P < 0 05) U. tmg CO, assimllaledidm’ leaf arealhr side; and Larry I? LaPre‘is a private environmental tg H,O lostidrn’ leaf areaihr consultant. Their work was supported by the Cali- fornia Iceberg Lettuce Research Board. CALIFORNIA AGRICULTURE. NOVEMBER-DECEMBER 1981 5 .