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Abstract

A collection of Lactuca saligna (P.I. 261653), although heterozygous, contained individuals that retard the development of cabbage loopers (Trichoplusia ni Hubner) compared with preferred hosts such as cultivars of lettuce and broccoli. Development of 1st instar to adult required 5 to 6 days longer on L. saligna than on broccoli. Comparable development of lst-instar larvae to 2nd instar required 32 hours more for larvae fed exclusively on leaves of L. saligna than for those confined to L. sativa cv. Hanson. Also, more significantly, in this test 26% of the larvae died before the 2nd-instar stage compared to 0% in the control.

Interspecific hybrids between L. saligna and L. sativa were produced. The F₁ plants were fertile to a limited extent. These interspecific crosses will allow an exchange of genes between the 2 species and make possible the development of lettuce cultivars resistant to the cabbage looper.

Five collections of L. saligna (other than P.I. 261653) were evaluated for their effect on looper development from 1st to 2nd instar. A significant delay in larval development was established for 2 entries (P.I. 253299 and P.I. 281876). However, they were more preferred than P.I. 261653 in a preference test. A collection of Lactuca species, consisting of 19 entries were tested for antibiosis. Two entries listed as L. perennis L. were uncovered where development from 1st instar to 2nd instar was suppressed. This result suggests that antibiosis for cabbage looper is not widespread in the genus, but probably fairly common in L. saligna as evidenced by plant to plant variation among numerous collections.

Azadirachtin (ATI), an insect growth regulator derived from extracts of neem (Azadirachta indica A. Juss) seed, was evaluated for the control of cabbage looper (Trichoplusia ni Hübner), diamondback moth (Plutella xylostella L.), and silverleaf whitefly (Bemisia argentifolii Bellows and Perring) in cabbage (Brassica oleracea L. Capitata Group) grown in southwestern Texas. In Fall 1992, ATI was tested with the a.i. at 0, 22, 33 and 44 g·ha^–1. In 1993, ATI was evaluated at 33 g·ha^–1 and in combination with M-Pede (1%, v/v), an organic insecticide based on potassium salts of fatty acids at 49%. Two commercial (Align and Neemix) and one experimental hydrogenated (LDF) ATI formulations were evaluated at 11 g·ha^–1 in 1994. Insect populations were monitored weekly before and after treatment application. Plant damage was evaluated immediately before harvest, and marketable yields were determined. In 1992, large (>6 mm long) and total cabbage looper counts were reduced by ATI compared with the nontreated control. Insect mortality was similar for all ATI rates tested in 1992. In 1993, ATI at 33 g·ha^–1 + M-Pede reduced the number of cabbage looper and diamondback moth larvae. ATI efficacy against cabbage looper and diamondback moth was enhanced when crop oil (polyol fatty acid esters with polyethoxylated derivatives) was tank-mixed with Align or LDF formulations in 1994. ATI did not reduce the number of silverleaf whitefly nymphs compared to the control. In all seasons, ATI-treated plants had lower insect-induced plant damage and higher marketable head weights than the nontreated control. Using ATI on lepidopterous pests appears to be beneficial for integrated pest management strategies.

Collard greens (Brassica oleracea var. acephala L.) were planted in the peripheries of cabbage (Brassica oleracea var. capitata L.) fields in the spring growing seasons of 1997 and 1998 to evaluate their effectiveness as a trap crop to manage the diamondback moth (DBM) [Plutella xylostella (L.)]. The numbers of DBM never exceeded the action threshold for application of insecticides in any of thefields that were completely surrounded by collards, but did exceed the action threshold in three of the fields without collards on four sampling dates in 1998. In both years, the numbers of DBM larvae in the collards exceeded the action threshold of 0.3 total larvae/plant in eight of nine fields. Larval counts in cabbage surrounded with collards were not significantly higher than in the conventionally planted cabbage, even though the number of pesticide applications was reduced in the former. The few pesticide applications in fields surrounded by collards probably targeted the cabbage looper [Trichoplusia ni (Hübner)], which was not impeded by the collards from infesting the interior cabbage. There was no significant reduction in marketability, and damage to cabbage was similar to that in fields where collards were planted and in fields where only conventional pesticides were used. The reduced number of pesticide sprays, as well as the high concentration of host larvae in the collards, may help maintain populations of natural enemies of DBM in the agroecosystem. Planting collards in field peripheries is a potentially effective tactic to manage DBM in cabbage.

Collard greens (Brassica oleracea var. acephala L.) were planted in the peripheries of cabbage (Brassica oleracea var. capitata L.) fields in the spring growing seasons of 1997 and 1998 to evaluate their effectiveness as a trap crop to manage the diamondback moth (DBM) [Plutella xylostella (L.)]. The numbers of DBM never exceeded the action threshold for application of insecticides in any of the fields that were completely surrounded by collards, but did exceed the action threshold in three of the fields without collards on four sampling dates in 1998. In both years, the numbers of DBM larvae in the collards exceeded the action threshold of 0.3 total larvae/plant in eight of nine fields. Larval counts in cabbage surrounded with collards were not significantly higher than in the conventionally planted cabbage, even though the number of pesticide applications was reduced in the former. The few pesticide applications in fields surrounded by collards probably targeted the cabbage looper [Trichoplusia ni (Hübner)], which was not impeded by the collards from infesting the interior cabbage. There was no significant reduction in marketability, and damage to cabbage was similar to that in fields where collards were planted and in fields where only conventional pesticides were used. The reduced number of pesticide sprays, as well as the high concentration of host larvae in the collards, may help maintain populations of natural enemies of DBM in the agroecosystem. Planting collards in field peripheries is a potentially effective tactic to manage DBM in cabbage.

Abstract

Resistance to Trichoplusia ni (Hubner), Pieris rapae (L.), and Plutella xylostella (L.) in cabbage [Brassica oleracea L. (Capitata group)] and cauliflower (B. oleracea, Botrytis group) was studied using a 5-parent diallel. Resistance was quantitatively inherited without undesirable linkages. When Plant Introduction (PI) 234599 (a glossy-leaved cauliflower) was used as a parent, narrow sense heritabilities of 22-47% were obtained for resistance. This resistance was maintained irrespective of plant age or presence or absence of alternate oviposition hosts. Plants with moderate tolerance only express it at maturity. The highest levels of resistance were transferred with difficulty into desirable type cauliflower and cabbage with slight bloom.

Abstract

F₁ progenies from a cross between Lactuca sativa L. breeding line 54671 and L. serriola L. PI 274372 (resistant to the cabbage looper, Trichoplusia ni (Hubner), averaged 42 ± 6 looper eggs per plant, compared to 213 ± 25 for the 54671 parent and 17 ± 4 for PI 274372. Two F₂ populations varied widely in plant damage inflicted by the resulting larvae when they were exposed to 4 releases of adult loopers but the damage distribution was skewed towards the resistant parent. Antixenosis of 16 F₃ progenies was independent of plant size (r ranged from 0.02 to 0.52) and of plant type (r ranged from 0.00 to 0.57).

Gluconasturtiin (2-phenylethyl glucosinolate), an aromatic glucosinolate, was used to evaluate the response of Chinese cabbage (Brassica campestris L. ssp. pekinensis) cv. Green Rocket to three and five cabbage looper (Trichoplusia ni Hubner) larvae per plant. Plants were harvested 0, 10, and 17 days after infestation. The change in gluconasturtiin content due to decreased light and leaf area removed was also studied. All samples were assayed for gluconasturtiin content using high performance liquid chromatography (HPLC). The gluconasturtiin content of plants subjected to five larvae/plant had a 59% increase, compared to noninfested plants 10 days after infestation. The effect of larval feeding was also dependent on harvest time. The levels of gluconasturtiin increased by 52% from the first harvest (prior to infestation) to the second harvest (10 days after infestation) in both larval feeding densities. Seventeen days after infestation (final harvest), gluconasturtiin content experienced a nonsignificant 6% decrease, compared to the previous harvest.

There were no differences in mortality, plant preference, or plant damage when diamondback moth (DBM) (Plutella xylostella L.) larvae were tested in no-choice and free-choice tests using leaf disks of resistant (`Green Glaze') or susceptible (`Vates', standard commercial cultivar) collards (Brassica oleracea Acephala group). No residuals of the pyrethrin insecticide Asana-XL (esfenvalerate) were detected 6 days after its application when DBM larvae were exposed to excised foliage for 72 hours. In a field test, more imported cabbage worm (ICW) (Pieris rapae L.) eggs were found on `Vates' treated with the insecticide than on nontreated Vates' or nontreated or treated `Green Glaze'. The fewest ICW, cabbage looper (CL) (Trichoplusia ni Hubner), and DBM larvae were found on the insecticide-treated cultivars. Fewer caterpillars were found on `Green Glaze' than Yates'. An additive effect of plant resistance and insecticide application lowered counts of DBM, ICW, and CL larvae. Percent parasitism of DBM by Diadegma insulare Cresson (Hymenoptera: Ichneumonidae) was lower on cultivars treated with the insecticide. Field plant damage ratings were higher for nontreated `Vates' and lowest for treated cultivars, but nontreated `Green Glaze' had a significantly lower feeding damage rating than nontreated `Vates'. Chemical name used: (S)-cyano (3-phenoxy phenyl) methyl-(S)-4 chloro-alpha (1-methylethyl) benzeneacetate [esfenvalerate (Asana-XL)].