Lettuce growth may be divided into three stages with regard to lepidopter- ous . The first occurs from germination to thinning, when the plant stand is established and made uniform by thinning. Lettuce may be planted by either the standard or precision method. Standard planting allocates a large num- ber of seeds to a unit area, and the resulting high-density stands are thinned. Precision planting allocates fewer seeds per unit area, resulting in lower seed costs and reduced labor re- quirements for thinning. The second (intermediate) stage be- gins immediately after thinning and continues until rosette formation, which marks the initial leaf formation of the lettuce head. During this period, the plant grows and establishes a leaf canopy that later aids in production of the head. The last stage, during which the head forms, enlarges, and matures, starts at rosette formation. We performed studies to determine the effect of lepidopterous larval infes- tations on lettuce plants during the two growth stages before rosette formation and to establish density treatment levels for commercial use. We also investigat- ed the effects of methyl and methomyl applications on photosynthe- sis and crop yield when pest densities were not economically significant dur- ing the intermediate growth stage. The studies were conducted at UC field stations: those on density treat- California lettuce, valued at $438.6 prevent damage, growers have generally ment levels in Imperial, Orange, and million in 1981, is grown mainly in the applied to maintain lepi- Fresno counties, and those on pesticide Imperial, Palo Verde, and Salinas val- dopterous larval densities at no more thresholds primarily in Orange County. leys, with smaller acreages in the San than 0.05 larva per plant (5 infested In all studies, iceberg head lettuce was Joaquin and Santa Maria valleys. To be plants per 100 plants examined) direct-seeded in double-row beds with marketable, lettuce heads must be pro- throughout the lettuce growing season, 40-inch centers; plants were sprinkler- tected against insect contamination. regardless of the pest’s or ’s irrigated to germination and then fur- Iceberg head lettuce is susceptible to effect on preheading plants. row-irrigated until harvest. - damage by several lepidopterous pests, Our studies have shown that densi- treated bait (1 pound active ingredient of which the cabbage looper, Trichoplu- ties of the major lepidopterous pests per acre) was applied to all plots during sia ni (Hubner), beet armyworm, Spo- infesting lettuce may fluctuate within a plant establishment for control of crick- doptera exigua (Hubner), tobacco bud- growing season and from one year to the ets and flea beetles. Plants were thinned worm, Heliothis virescens (F.), and corn next in the same area. Scheduled pesti- to 10,890 seedlings per acre at the three- earworm, Heliothus zea (Boddie), are cide applications made without regard leaf stage. Experimental replicates con- predominant in California. Less com- to the insect densities in the field may sisted of four beds, 50 feet long, and all mon lepidopterous species, such as the be unnecessary and may ultimately re- treatments were replicated four times. alfalfa looper, Autographa californica duce lettuce yields. Previous studies In the density treatment studies, 25 (Speyer),may also infest lettuce. Before revealed that the insecticides methyl plants from the center two beds of each thinning, seedlings may be injured by parathion, methomyl, and plot were inspected twice weekly for all flea beetles, Epitrix sp., and crickets, can reduce leaf photosynthetic rates lepidopterous larvae present between Gryllus spp. after a single application at recommend- germination and thinning, and once Previous studies on lettuce insect ed rates (California Agriculture, Novem- weekly thereafter. pests have largely been confined to field ber-December 1981) and that yield evaluations of insecticides. No quantita- losses are possible following multiple Seedling stage tive relationships have been established applications. A “pesticide threshold” is In 1978, half of the plots were planted between insect densities during the needed indicating the maximum num- by the standard and half by the preci- growing season and subsequent yield ber of applications permissible for any sion method (seeds in pairs at 2-inch reductions. The lack of such informa- one insecticide at recommended rates to and at 10-inch intervals, respectively). tion providing reliable density treat- avoid economically significant yield re- Insecticides were applied before thin- ment levels has impaired insect pest ductions caused by subtle phytotoxic ning to maintain larval densities below management programs for this crop. To properties of the compound. preestablished levels of 0, 0.05, 0.1 and

6 CALIFORNIA AGRICULTURE,JANUARY-FEBRUARY 1984 0.2 larva per plant. After thinning, in- Yields also were smaller when larval olds based on mean lettuce yields from secticides were applied when larval densities were maintained at or near 0 the Orange and Imperial County stud- density was 0.05. In an untreated con- per plant during this growth period. ies. Yield was significantly lowest trol, pest populations were permitted to Significantly lower yields from heavily where lepidopterous larvae were un- develop naturally until thinning, after treated plots may have resulted from checked. Yield increased about 30 per- which the control was treated the same insecticide-induced suppression of cent at levels of 2 and 5 larvae per plant. as the other treatments. The insecti- physiological processes that contribute Optimum yields occurred under treat- cides methomyl or (0.9 or 1 to the vegetative growth of the lettuce ments to maintain levels of 0.5 to 1 larva pound active ingredient, respectively, plant and, subsequently, lettuce head per plant. Maintaining the lowest level in 60 gallons of water per acre) were weight. (O), which decreased yield, required an used interchangeably. We devised a hypothetical yield re- average of three more insecticide appli- Yields of lettuce heads (harvested on sponse curve (fig. I), indicating poten- cations than maintaining the levels of December 27, trimmed to commercial tial “economic” and “pesticide” thresh- 0.5 and 1 larva per plant. Optimum standards, sized, and graded) showed no significant differences between the two planting methods. An equal number of insecticidal applications was required to Insecticide applications required to maintain common lepidopterous larval populations below various protect standard- and precision-planted density treatment levels on lettuce and corresponding yields seedlings from larval feeding damage. Density Number of. insecticide Keeping larval infestation at 0 as com- Location treatment level applications at stage’: Yieldt pared with 0.2 larva per plant gave no and date (larvaelplant) One Two Three (cartonslacre# yield advantage. However, significant Orange County 0.0 18 4 3 336.5 ab yield increases were associated with 1977 0.5 1 1 4 380.7 ab 1 .o 1 1 4 421.1 ab treatments made to maintain a level of 2.0 1 1 4 444.8 a 0.1 larva per plant as compared with the 5.0 1 0 3 439.8 ab untreated check in precision-planted Check 1 0 0 237.7 b plots (yields of 647 as opposed to 185 Imperial County 0.0 26 6 5 425.2 b cartons per acre, respectively). 1977 0.5 2 2 5 518.1 ab 1 .o 2 2 6 559.4 a Intermediate stage 2.0 2 1 6 429.6 b 5.0 2 1 5 425.7 b During the intermediate growth peri- Check 2 0 0 20.9 c od, in studies from 1977 through 1979, 1978 0.0 1** 5 4 616.6 b insecticides were applied to maintain 0.5 1 1 3 634.2 ab total larval densities below preestab- 1.o 1 1 4 715.6 a lished levels of 0, 0.5, 1, 2, or 5 larvae 2.0 1 0 3 470.3 b 5.0 1 0 3 542.4 ab per plant. After rosette formation was 70 Check 1 0 0 461.0 b percent complete, all plots were treated Fresno County 0.0 0 7 2 1167.5 ab when 0.05 larva per plant was found. 1979 0.5 0 2 3 1384.1 a The same insecticides and rates as in 1 .o 0 2 2 1265.9 ab the seedling study were compared with 2.0 0 1 3 1195.0 ab an untreated control. 5.0 0 0 2 1267.5 ab Check 0 0 0 1041.7 b Two or more larvae per plant between * Stage One: germination to thinning. Stage Two: thinning to rosette formation. Stage Three: after rosette formation; thinning and rosette formation caused density treament level changed to 0.05 larva per plant at this stage. feeding damage and subsequent reduc- t Numbers followed by same letter are not significantly different by Duncan’s multiple range test (P>0.05). tion in yield when compared with levels + Carton of lettuce weighs 63.8 pounds. 5 Insecticide applied to all treatments to protect seedlings against crickets and flea beetles. of 0.5 and 1 larva per plant (see table). .. Insecticides applied to all treaments to protect seedlings against cutworms.

CALIFORNIA AGRICULTURE, JANUARY-FEBRUARY 1984 7 yields may not always occur at these applications of methyl parathion and latter two levels, however, because of subsequent decreases in lettuce yield variations in pest population pressures are unknown. and damage to the crop by the various Pesticide-induced yield reductions species. are not limited to lettuce and have been reported in apples, Bartlett pears, straw- Harvest and Pesticide threshold berries, and cotton. Our results indicate Field studies on the influence of the that lettuce yield reductions caused by number of insecticide applications on the application of methyl parathion re- lettuce yields were conducted during sulted from reduced head weight and the intermediate growth stage, when increased plant bolting. We calculated growers have some flexibility in their the total effect of methyl parathion and control programs. Methyl parathion and methomyl applications on these two methomyl(1 and 0.9 pound active ingre- factors, expressed as yield per acre, dient per acre, respectively) were ap- based on an initial stand density of plied two. four, six, and eight times each 25.490 plants per acre (fig. 2). A maxi- ujube, or Chinese date, is a deciduous during the intermediate growth period. mum yield reduction of less than 7 Iruit tree of tropical and subtropical In all treatments, both insecticides were percent was estimated for plants that origin, now grown primarily in home applied twice before thinning and twice received a total of 12 applications of gardens in California and Florida. following rosette formation. An untreat- methomyl as compared with the un- Among problems that have limited de- ed check was included. treated check. Reductions of about 57 velopment of jujube as a commercial Lepidopterous larval pests remained percent were estimated for replicates crop in California are several factors below one larva per plant during the that received methyl parathion applica- related to harvesting and postharvest intermediate growth period in all treat- tions. A yield reduction of 16 percent handling. These include variation in ments. Plants treated with methyl par- was estimated for three applications of ripening time among fruits, failure of athion produced lettuce heads that methyl parathion. green fruits to ripen after harvest, and weighed less than those in the untreat- poor storageability of ripe fruits on the ed check. Reductions in head weight Summary tree. To help overcome these limita- and density were correlated with the Provided that lettuce plants are pro- tions. we conducted studies on composi- number of applications of methyl par- tected during the heading stage, it is not tional changes associated with matura- athion that plants received during the necessary to maintain the crop insect- tion and ripening and on optimum growing season. Plants that received free during the entire growing season to postharvest handling temperatures for over three methyl parathion applica- achieve high yields and quality pro- transport and storage of these fruits. tions produced poorly formed (spongy) duce. Our results indicate that the sea- heads unsuitable for marketing as com- son-long, low-pest-density approach Fruit characteristics pared with the tightly wrapped heads may cause yield losses resulting from The fruit of the jujube, Zizyphus ju- produced in the check and methomyl subtle phytotoxic properties of the in- juba, is a drupe that is round to oblong plots. The percentage of lettuce plants secticide applied. Density treatment in shape and varies from cherry-size to that bolted before a marketable head levels of 0.1 and 1 larva per plant during plum-size, depending on the cultivar. Its formed increased with the number of the seedling and intermediate growth skin color changes from green to whit- methyl parathion applications (from 5 to stages. respectively, should permit pro- ish green to brown during ripening. The 57 percent at 0 to a total of 12 applica- duction of marketable lettuce while re- thin edible skin surrounds a whitish tions during the entire growing season). ducing the number of insecticide appli- flesh and a two-seeded stone. Fruits In contrast. methomyl applications cations made normally. ripen in September. Later, as they dry, were correlated with increased lettuce With regard to methyl parathion ap- the fruits wrinkle and change in color head weight without influencing head plications, a pesticide threshold of no from reddish brown to dark brown. density. However, a positive correlation more than three applications is suggest- They can be eaten fresh, dried, or can- was found between the percentage of ed for use on lettuce grown in southern died. plants that bolted before formation of a California to avoid insecticide-caused CVhen ripe, jujube fruits contain more marketable head and the number of yield reductions greater than 20 per- sugars and less acidity than most other methomyl applications per season, al- cent. This threshold may vary, depend- fruits. which give them a sweet and sub- though the correlation was not as strong ing on plant vigor and maturity, chemi- acid taste. They have a somewhat pithy as under methyl parathion applications. cal adjuvants used with the methyl and relatively dry flesh, since they con- In additional studies at the UC Citrus parathion, and environmental condi- tain less water than other fresh fruits. Research Center, Riverside, on effects tions. of various methyl parathion and metho- my1 application rates, both materials Marshall W. johnsan, former Post- raduate Re Compositionof ripe jujube fruits decreased lettuce leaf photosynthetic search Assistant, Universitv of California. River: (on fresh weight basis) rates 24 hours after application. Reduc- side, is Assistant Professor of Entomolog , Univer- sity ofHawaii at Manoa: Ken Kido is StafyResearch tions were of short duration, and photo- Associate, Cooperative Extension, University aj Component Range synthetic rates were not significantly California, Riverside: Nick C. Toscano is Program different from those of untreated plants Director of Pest Management and Entomologist, Water ("10) 71.7-74.0 four and eight days after application. Cooperative Extension. UC, Riverside: Robert A. Soluble solids (YO) 21.9-23.0 Van Steenwyk is Entomologist, Cooperative Exten. Total sugars (Oh) 20.2-21.5 However, the temporary reductions sion, UC. Berkeley; Steve C. Welter is Assistanl Reducing sugars ("/a) 16.9-18.0 Professor of Entomology, San Diego State Universi. were correlated with increases in insec- ty. San Diego; and Norman F. McCalley is Farm PH 4.68-4.71 ticide application rates. Currently, pos- Advisor, UC Cooperative Extension. Montereq Titratable acidity (%) 0.20-0.23 County. The authors acknowledge the assistance oj Total phenolics (mg/lOO g) 27 0 - 3 0 5 sible relationships between the suppres- William W. Barnett. Area IPM Specialist, Cooper. Total ascorbic acid (mg/lOO g) 500-560 sion of photosynthetic rates by ative Extension, Fresna County.

8 CALIFORNIAAGRICULTURE. JANUARY-FEBRUARY 1984