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

(Gupta and Thorsteinson 1960; reviewed in Sarfraz et al.2006).The diamondback moth was discovered on sugar snappeas (Pisum sativum, Fabaceae) in the Naivasha regionof Kenya in 1999 (Loehr 2001). The strain collectedfrom this infestation (DBM-P) is able to completedevelopment on both pea plants and the ancestralBrassica plants, indicating that a host range expansionrather than a host shift has occurred (Loehr and Gathu2002). Adult females of the pea strain retain a preferenceto lay eggs on cruciferous plants, but they are more likelythan other strains to lay eggs on pea plants in choice andno-choice experiments (Henniges-Janssen et al. 2011).Loehr and Gathu (2002) proposed that survival on peawas related to “the ability to initiate feeding without thenormal stimuli present in crucifers.” Subramanian andLoehr (2006) conducted an experiment of host plantchoice of neonates, and we would like to expand fromthis work. Thus, the additional aims of this study are tobetter characterize the behavioral componentscontributing to the host range expansion in the pea strainand determine the inheritance of those components.MATERIALS AND METHODSPetri dish assaysObservational assays of a duration of 5 minutes wereconducted to measure attraction to the two different hostplants in choice and no-choice scenarios. Observationswere made every 30 seconds and included whether aneonate reached a particular leaf disc and whether aneonate left its respective sector (Figure 1). In this way,host searching behavior can be characterized by bothspeed and level of directed movement.walk (Figure 1C). This was done because some neonatesprefer to hold an inverted orientation, and the smallvertical space allowed neonates to do so. In the center ofthe arena was placed a leaf disc (14mm) of eithercabbage or pea, and a ring of moist filter paper (width1cm) was placed around the outer edge both for moistureand as a deterrent to neonate escape. Neonates wereplaced (15mm) away from the edge of the arena andwere observed four at a time per family.ChoiceThe arena was set up similarly to the no-choiceexperiments, but with neonates beginning the assayinside a 15 mm circle in the center and with leaf discsplaced 3mm from the edge (Figure 1B). Two cabbageleaf discs and two pea leaf discs (12mm) were placedopposite each other, respectively. Two or four neonatesper family were observed at a time; observations are thesame as described above. Most neonates made a choicewithin the 5 minute assay; the 7 of 76 total neonates thatdid not make a choice were excluded from analyses. Inthese choice assays, the petri dish arenas were checked24 h after the initial observation to see whether theproportion of neonates on the respective leaf discs wasthe same.Vial assayBased on results from the short term petri dish assays, anassay of longer duration on individual neonates wasdevised. Plastic vials with a height of 2.5 cm and widthof 1.2 cm contained a leaf disc of cabbage and pea ateither side. Leaf discs were randomized with regard toposition in the vial. Neonates were placed on the side ofthe vial between the leaf discs and were checked 24 hlater for their choice. Vials were placed on their side forthe assay so that preferred orientation of neonates wasnot a confounding factor in the experiment.To test applicability of this assay for use as a marker,results were compared to survival data in two ways.First, the percentage of neonates per family that chosethe pea disc was compared to the percent of individualsper family that were able to complete development topupation on a diet of pea leaves. Individuals weresiblings and not the same ones used to test survival. In aseparate set of replicates, however, larvae were reared topupation on their choice as neonates.Figure 1. Diagram of the petri dish assays (notnecessarily to scale). A) No choice assay; B) Choiceassay; C) Profile view of the no choice assay.No choiceThe arena consisted of an inverted lid, into which wasplaced the bottom of the petri dish (Figure 1A), leavingabout 1-2mm vertical space in which the neonates couldRESULTS AND DISCUSSIONPetri dish assaysNo choiceThere was no difference in attractiveness of the two hostplants between the two host races. Equal proportions ofthe cabbage strain and the pea strain go to both thecabbage (χ 2 =0.066, P=0.797, n=52) and pea leaf discs(χ 2 =0.265, P=0.607, n=52) in the 5 minute assay (Figure2). In a test for level of directed movement, i.e. howmany neonates left their sector, the pea strain exhibited4 AVRDC - The World Vegetable Center