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.