Choice and No-choice Tests for Larvae

Two studies were performed under the foregoing laboratory conditions to determine larval melonworm feeding preferences: choice tests and no-choice tests. All larvae (5-d-old second instar) were obtained from the previously described laboratory colony and originated from the same cohort. Leaf discs for choice and no-choice tests were cut from young, fully expanded leaves on greenhouse plants using a 2.5-cm-diameter cork borer with care taken to avoid major leaf veins. For the choice test, twenty 13.5-cm-diameter Petri dishes were used with each dish serving as a replication. Within each dish, a leaf disc from each of the four crop cultivars was placed randomly. One 5-d-old melonworm larva was placed in the center of each Petri dish, which was covered with a lid and left undisturbed. To determine larval preference for each cultivar and the amount of feeding based on leaf area removed, all leaf discs with feeding damage were removed after 17 h, then scanned with a leaf-area meter (LI-COR portable area meter LI 3000, Lambda Instrument Co., Lincoln, NE).

The no-choice test had a 2.5-cm-diameter leaf disc per Petri dish and used four 8.5-cm-diameter Petri dishes, one per cultivar, for each replication. The no-choice study had five replications each time; hence, 20 total Petri dishes were used, which was repeated four times in 10-d study period. Hence, the total replication for each host was 20. Otherwise, experimental procedures were the same as with the choice study.