Remove, control or exclude native predators

Supporting evidence from individual studies

1. A replicated, paired, controlled study in 1964–1965 in a fen in Cambridgeshire, UK (Duffey 1968) reported that excluding native predators increased the survival of large copper Lycaena dispar batavus caterpillars. Results were not tested for statistical significance. On plants which were caged to exclude birds and mammals, the survival of large copper caterpillars was 73% (79/108 survived), compared to 4% (11/300 survived) on plants without cages. Data from four caged plants were excluded as the caterpillars abandoned them after eating all of the leaves. The author reported that on plants kept in cages which excluded birds, mammals and parasitic insects, 297/354 (84%) caterpillars survived. Four batches of 20 great water dock Rumex hydrolapathum plants were selected. In each batch, four plants were >100 cm tall, and 16 were 50 cm high. In May 1964, three or 12 large copper caterpillars were placed onto each plant, and half of the plants in each batch were covered with a 6-mm plastic mesh cage to exclude birds and mammals. In July 1964, all surviving caterpillars and pupae were counted. In 1965, a total of 354 caterpillars were reared in six muslin cages to exclude birds, mammals and parasitic insects (no further details provided).

   Study and other actions tested

   Referenced paper

   Duffey E. (1968) Ecological studies on the large copper butterfly Lycaena dispar (Haw.) batavus (Obth.) at Woodwalton Fen National Nature Reserve, Huntingdonshire. Journal of Applied Ecology, 5, 69-96.

2. A replicated study in 1996–1998 in two peatland sites in Wrexham and Shropshire, UK (Joy and Pullin 2001) found that when cotton-sedge tussocks Eriophorum vaginatum were caged to exclude vertebrate predators, overwinter survival of large heath butterfly Coenonympha tullia caterpillars was no higher than when the tussocks they were on were uncaged. There was no difference in survival between caterpillars on caged and uncaged tussocks on Fenn’s Moss (1997 caged: 12 of 40, uncaged: 16 of 40) or on Whixall Moss (1997 caged: 24 of 40, uncaged: 26 of 40; 1998 caged: 23 of 40, uncaged: 20 of 40). In winter 1996–1997 one study plot was located at Whixall and one plot at Fenn’s, but in 1997–1998 two plots were located at Whixall only. In September 1996 and 1997, twenty captive-reared large heath caterpillars were put on each of four cotton-sedge tussocks in that year’s plots (two plots/year and a total of 80 caterpillars/plot). In October 1996 and September 1997, mesh cages of 50 x 50 x 87 cm were placed over two of the four tussocks in each of that year’s plots. The mesh had 0.5 cm² gaps to exclude vertebrate but not invertebrate predators. In April 1997 and May 1998 the tussocks were searched thoroughly for surviving caterpillars.

   Study and other actions tested

   Referenced paper

   Joy J. & Pullin A. (2001) Field studies on flooding and survival of overwintering large heath butterfly Coenonympha tullia larvae on Fenn’s and Whixall Mosses in Shropshire and Wrexham, UK. Ecological Entomology, 24, 426-431X.

3. A replicated, paired, controlled study in 2005–2007 in two rainforest blocks in western Kenya (Mbahin et al. 2010) found that Boisduval silkworm Anaphe panda eggs and caterpillars protected with a net had a higher survival rate than unprotected caterpillars. More silkworm egg clusters survived to pupation when they were protected with a net (114/126 clusters, 90%) than when they were unprotected (66/95 clusters, 69%). The survival of individual caterpillars through to pupation was higher when their egg cluster was protected with a net (16,645/25,571 caterpillars, 65%) than when it was unprotected (6,068/32,411 caterpillars, 19%). In 2005–2007, across one 380-ha natural forest containing only native trees, and one 415-ha modified forest containing native and non-native trees, 150 mitzeeri Bridelia micrantha trees with ≥2 silkworm egg clusters were chosen. On each tree, one egg cluster was covered with a 1.5 × 1.5 × 2-m net sleeve tied closed on the branches, one egg cluster was left uncovered, and any additional egg clusters were removed. Protected caterpillars were moved to new branches of the same tree 2–3 times during development to maintain their food supply. From June 2005–June 2007, the number of surviving caterpillars in each cluster was counted twice/week. Only groups where some individuals survived to pupation were included in the analysis of individual survival.

   Study and other actions tested

   Referenced paper

   Mbahin N., Raina S.K., Kioko E.N. & Mueke J.M. (2010) Use of sleeve nets to improve survival of the Boisduval silkworm, Anaphe panda, in the Kakamega Forest of western Kenya. Journal of Insect Science, 10, 1-10.

4. A replicated, randomized, paired, controlled study in 2011–2012 in a pine forest in North Carolina, USA (Aschehoug et al. 2015) found that excluding native predators increased the survival of Appalachian brown Satyrodes appalachia eggs, caterpillars and pupae. In plots where predators were excluded, the survival of Appalachian brown eggs (48–94%) and caterpillars and pupae (35–60%) was higher than on plots left open to predators (eggs: 12–74%; caterpillars and pupae: 7–37%). In May 2011, four 30 × 30 m plots in each of four blocks were established. From 15 May–15 June and 7 July–7 August 2012, two potted sedge Carex mitchelliana plants were placed in the centre of each plot. To exclude predators, one plant/plot was enclosed in a fine mesh fabric cage, and had a 15-cm band of sticky resin painted on its pot. Each plant had a known number of butterfly eggs, laid by caged wild-caught females prior to placement. The number of eggs on each plant which survived after 48 hours was counted. In addition, in each of six arenas/plot (created from polyethylene food drums), centred on mature sedge, five captive-reared caterpillars (first to third instar) were released and the number of emerging adults was counted. Three arenas were enclosed with tulle netting, and potential predators (mainly spiders and ants) were removed prior to the release of caterpillars.

   Study and other actions tested

   Referenced paper

   Aschehoug E.T., Sivakoff F.S., Cayton H.L., Morris W.F. & Haddad N.M. (2015) Habitat restoration affects immature stages of a wetland butterfly through indirect effects on predation. Ecology, 96, 1761-1767.

5. A replicated, paired, controlled study in 2014 in a managed park in Georgia, USA (Majewska et al. 2018) found that excluding terrestrial predators did not increase the survival of monarch Danaus plexippus caterpillars. The survival of monarch caterpillars from first to fifth instar was similar on plants where terrestrial predators were excluded and on plants without predator exclusion (data presented as model results). Swamp milkweed Asclepias incarnata plants were grown in greenhouses, and placed outside in July 2014. Plants were paired at 28 locations, six within grassland plots planted with native species, six within grassland plots planted with exotic species, and 16 in open grassland and forest edges. One plant/pair was placed in a 2-litre tub of water with Tanglefoot^TM applied to the rim of the pot to exclude non-flying predators (mainly ants) and the other was placed directly on the ground. Each plant was surrounded by fencing to prevent deer browsing. In October 2013, monarch butterflies were collected on migration, and reared for two generations. In July 2014, two to four first-instar caterpillars were placed on each milkweed and monitored daily until they reached the fifth instar.

   Study and other actions tested

   Referenced paper

   Majewska A.A., Sims S., Wenger S.J., Davis A.K. & Altizer S. (2018) Do characteristics of pollinator-friendly gardens predict the diversity, abundance, and reproduction of butterflies? Insect Conservation and Diversity, 11, 370-382.