DOI: 10.5061/dryad.1jwstqk20
title: Data from: Host plant phenology drives risky larval dispersal in an outbreaking insect defoliator

description: Dispersal away from natal sites allows individuals to find suitable
 foraging sites to complete their development and successfully reproduce.
 However, the decision to disperse is risky for young larval Lepidoptera
 because mortality rates as they move from source to sink sites are high.
 Factors that drive this risky dispersal behaviour in forest landscapes by
 early instar larvae of outbreaking defoliators such as the eastern spruce
 budworm are not well understood.  We assessed dispersal behaviour
 of young larvae in relation to resource availability in terms of spring
 budburst phenology of primary and secondary host species, balsam fir and
 black spruce, respectively. We tested whether tree species and presence of
 suitable feeding sites influenced dispersal away from source branches and
 subsequent redistribution of insects. Laboratory experiments showed that
 dispersal is an active behaviour during which larvae disperse away from
 source trees without expanded buds, regardless of species. Establishment
 on sink branches was highest when they possessed accessible buds and the
 source did not. In the field, larval dispersal was higher from black
 spruce than from balsam fir and larval establishment was more persistent
 on balsam fir. Ballooning larvae settled more readily on balsam fir than
 on black spruce. Under current climatic conditions, the decision to
 disperse occurs before budburst of either host species. In this situation,
 larvae dispersed preferentially away from black spruce because old needles
 on this species are too tough for larval mining, whereas old foliage of
 balsam fir can be mined for sustenance and refuge while larvae await
 budburst. While black spruce would be a suitable host species after its
 buds expand, phenological defenses drive larval dispersal away from this
 host plant. These findings are the first to show how risky dispersal
 behavior in larval Lepidoptera is facultative and is determined by local
 food availability.
description: # Host plant phenology drives risky larval dispersal in an outbreaking
 insect defoliator
 [https://doi.org/10.5061/dryad.1jwstqk20](https://doi.org/10.5061/dryad.1jwstqk20) ## Description of the data and file structure [Access this dataset on Dryad](https://doi.org/10.5061/dryad.1jwstqk20) We assessed dispersal behaviour of young spruce budworm larvae in relation to spring budburst phenology of primary and secondary hosts, balsam fir and black spruce, respectively. We tested whether tree species and presence of suitable feeding sites influenced dispersal away from source branches and subsequent redistribution of insects. Laboratory experiments showed that dispersal is an active behaviour during which larvae disperse away from source trees without open buds, regardless of species. Establishment on sink branches was highest when they possessed open buds and the source did not. In the field, larval dispersal was higher from black spruce than from balsam fir and larval establishment was more persistent on balsam fir. The decision to disperse occurs before budburst of either host species. Larvae disperse preferentially away from black spruce whose old needles are too tough for larval mining, compared to balsam fir that can be mined for sustenance and refuge while larvae await budburst. While black spruce is a suitable host species after its buds expand, phenological defences drive larval dispersal away from this host plant. ## Description of the data and file structure Abundance_f.csv file - Data are from the field experiment. They give counts of spruce budworm population density on sampled branches of balsam fir and black spruce trees. L2 and L4 refer to the second and fourth larval stages that were sampled. Data sheet columns are: replicate / ID / L2 / L4 / total dispersing larvae / ratio of L2 / ratio of L4 Density 2020.csv file - Data are from the field experiment and represent the number of larvae on caged and uncaged (reference) branches during the dispersal phase in black spruce and balsam fir. The number of buds indicate resource availability on the tree. Data sheet columns are - Site number / species / Caged or Ref code / Treatment (caged or ref) / no. buds / no. larvae / branch wt / buds per unit wt / larvae per unit wt Lab exp data.csv file - This is data from the lab experiment. Treatments are in Figure 1 in the article. Counts are number of larvae that moved from one branch to another in each treatment. Phenology represents budburst stage of branch in the season. Data sheet columns are: Treatment / Tank / Source Species / Source buds / Sink Buds / Sink species / Total larvae installed / No. dispersed / Established / On balsam fir / On black spruce / Survived / Dead / Lost / MIA / Feeding source / Feeding sink / phenology / proportion dispersed. The first two data files represent data collected from the same trees in the forest. The lab experiment file is independent of the other two files. ## Code/Software All code is in R. ### Files and variables #### File: abundance\_f.csv **Description:** Data from field experiment ##### Variables * tree: Branch sampled from balsam fir or black spruce tree * ID: replicate * sp: species (balsam fir BF / black spruce BS) * L2: second instar larvae; units in counts * L4: fourth instar larvae; units in counts * Sum dispersing larvae: Total larvae that dispersed; units in counts * ratio L2: proportion of L2 that dispersed; no units * ratio L4: proportion of L4 that dispersed; no units #### File: density\_2020.csv **Description:** Data from field experiment ##### Variables * site: Tree species pairs selected at this spot (replicates) * sp: species; balsam fir and black spruce * cr: caged (C)  or reference (R) branch * trtment: treatment * buds: number of buds on branch; units in counts * larvae: number of larvae on branch; units in counts * branch weight: branch weight; units in grams * buds.by.weight: buds per unit weight; units in counts * larvae.by.weight: larvae per unit weight; units in counts #### File: lab\_exp\_data.csv **Description:** Data from lab experiment ##### Variables * Treatment: One of seven treatments from Fig 1 of article * Tank: Tank number (replicate) * Source_Sp: Species on which larvae were placed (balsam fir BF or black spruce BS) * Source_buds: Whether or not buds were present on source branch * Sink_buds: Whether or not buds were present on sink branch * Sink_sp: Sink species (balsam fir BF or black spruce BS) * Total: Total larvae installed; units in counts * Dispersers: Number of larvae that moved away from branch they were installed on; units in counts * Established: Number of larvae that established on sink branch; units in counts * On_BF: Number of larvae on balsam fir; units in counts * On_BS : Number of larvae on black spruce; units in counts * Survived: Number of larvae that survived; units in counts * Dead: Number of larvae that died; units in counts * Lost: Number of larvae lost; units in counts * MIA: Number of missing larvae; units in counts * Feeding_source: Number of larvae feeding on source branch; units in counts * Feeding_sink: Number of larvae feeding on sink branch; units in counts * Phenology: Budburst stage of branch; categorical variable - early, mid and late  * Proportion dispersed: Proportion that left source branch and settled on sink branch; no units ## Description of software files Fieldexperiment.R - R code for analysis of data from the field experiment; used to analyse data from files abundance_f.csv and density_2020.csv. Test.R - R code for analysis of data from the lab experiment; used to analyse data from file lab_exp_data.csv ## Access information Other publicly accessible locations of the data: * None Data was derived from the following sources: * Field and lab experiments.