An okra-leaf line of cotton, Gossypium hirsutum L., designated La-Okra-2, has sustained less seed damage as used by pink bollworm (PBW), Pectinophora gossypiella (Saunders), than the normal-leaf equivalent, 'Stoneville 213'. On the other hand, La-Okra 5-5 has sustained as much damage as the normal-leaf equivalent, 'Stoneville 213'. A third okra-leaf, normal-leaf pair, Aub-Okra 149 and 'TH-1491, both sustained low seed damage.

The lower seed damage in La-Okra-2 is caused by reduced penetration of bolls by 1st instars of PBW. We concluded, from detailed field studies, that microclimatic differences in the canopies of okra-leaf and normal-leaf cottons are not sufficient to cause the reduction in larval penetration. Results suggest, instead, that the reduced penetration is caused by some intrinsic property of the infestable boll.

In the laboratory, we placed five 1st-instar PBW on each of five 12-day-old bolls of the two okra-leaf and normal-leaf cottons in Stoneville 7A and Stoneville 213 background. We observed larval behavior every 10 min for 70 min. We repeated this procedure twice a week for 6 weeks. The only difference that was correlated with field results was that fewer larvae penetrated bolls of La-Okra-2 than those of Stoneville 7A for the first 50 min of observation; La-Okra 5-5 and Stoneville 213 did not differ in this respect. Other differences were as follows: 1) mean larval penetration was significantly lower on bolls of the okra-leaf, normal-leaf pair in Stoneville 7A background than in Stoneville 213 background; 2) more larvae chewed, without penetrating, bolls of okra-leaf cottons in both Stoneville backgrounds, but mean numbers were higher in the Stoneville 7A background; 3) more larvae crawled off the bolls of both cottons in Stoneville 213 background. No differences were observed in the number of larvae crawling, resting, or dead.

Thickness and compressive strength of carpel walls were measured for all three okra-leaf, normal-leaf pairs. Bolls of La-Okra-2 had thinner carpel walls than those of Stoneville 7A, while bolls of La-Okra 5-5 had thicker walls than those of Stoneville 213, a result opposite to what would have been expected if thicker carpel walls had reduced larval penetration. Compressive strength did not differ significantly in the four Stoneville cottons. Bolls of Aub-Okra-149 had significantly thicker carpel walls and higher compressive strength than those of La-Okra-2 or La-Okra 5-5 and bolls of TH-149 had significantly higher compressive strength than bolls of both Stoneville cultivars.

The resistance of La-Okra-2 is apparently caused by a physical or chemical property of the boll surface. The low seed damage observed in both TH-149 and Aub-Okra-149 could be caused by an increased physical barrier to larval penetration.