Management of the Diamondback Moth and Other Crucifer Insect ...

There was no difference in survival between whether aneonate chose cabbage or pea (χ 2 =0.28, df 29, P=0.595;Figure 7).assay design tests multiple neonates in fewer replicates ina petri dish arena.Figure 5. Proportion of neonates selecting thecabbage disc in the 24 h vial assay; the proportionchoosing pea is equal to the inverse of thosechoosing cabbage. The parental cabbage and peastrains significantly chose both cabbage and pea,respectively, in a Chi square test of equalproportions. The F1 hybrids in crosses of bothdirections did not show a significant preference,although offspring of a pea strain mother andcabbage strain father showed a slight, marginallysignificant preference for cabbage (P=0.06).Figure 6. Positive relationship between percent ofneonates choosing the pea disc in the 24 h vial assayand percent survival of their siblings (r=0.494;P=0.003). Parental cabbage strain families aredenoted with black circles, parental pea strainfamilies are denoted with open circles, and F 1 hybridfamilies are denoted with gray circles.Figure 7. Percent survival to pupation on whicheverhost the larvae chose as a neonate in the 24 h vialassay. Results are pooled by chosen host plant;there is no difference in survival between cabbageand pea (P=0.595).Comparing the assaysGustatory or physical cues may then influence whether aneonate remains on the host plant and initiates feeding.Loehr and Gathu (2002) suggest that the adaptation topea is largely dependent on whether the neonates willmine on pea leaves. Acceptance of alternate hosts can beenhanced by the application of sinigrin to leaves (Guptaand Thorsteinson 1960); in fact, incorporation of sinigrinand other crucifer compounds makes artificial mediapalatable to diamondback moth larvae (Thorsteinson1953). Sinigrin application to pea leaf discs has beenshown to increase its palatability to older larvae (vanLoon et al. 2002; Gupta and Thorsteinson 1960), andwhile it has not yet been tested on neonate larvae, onewould expect the same effect.While there does appear to be genetic control of choiceof host plant by neonates, the vial assay could be alteredin a way that promotes a clearer separation ofphenotypes. One aspect to consider is the likelihood of alarva to be able to make a choice in the field, given theirlimited mobility. One example of a behavioral assay thatmay be more reflective of nature is a “passing-over test”,where neonates are examined for whether they continuesearching for a host after passing over another host (Stuhlet al. 2008). An assay testing whether naïve neonates willreject a pea leaf disc is currently underway.The 24 h vial assay has potential for use as adiscriminating marker trait. Most importantly, individualneonate choice in the vial assay is reflective of theirability and their siblings’ ability to survive on pea.However, one might expect, based on the results of thepetri dish assay that pea strain neonates would show nopreference, but this effect was not observed. Thepercentage of both strains choosing pea is higher thanthat reported in Subramanian and Loehr (2006), but theirCONCLUSIONChanges in larval behavior can be subtle to detect, butclearly there was enough variation in the field to producenoticeable effects of damage on peas from herbivory bydiamondback moth larvae (Loehr 2001). The results fromexperiments on neonate choice demonstrate theimportance of the scale on which the plant cues affectbehavior. Neonates initially went to leaf discs of the hostplants, but physical or gustatory mechanisms likely6 AVRDC - The World Vegetable Center