Enemy release hypothesis

Not to be confused with Class enemy.

The enemy release hypothesis (ERH) proposes that plants and animals introduced to a new region succeed because they have left behind the herbivores, parasites and diseases that held them in check at home.¹ It is among the most widely invoked explanations of biological invasions, and among the most disputed. Biogeographical comparisons mostly find that introduced species have fewer kinds of enemy abroad than at home, while studies of invaded communities find them no less attacked than the native species around them.²

Giant hogweed looked like a clear case. In its native Caucasus it grows as scattered plants in species-rich meadows;³ in Europe it is one of the most aggressive plant invaders.⁴ When the hypothesis was put to the test in the early 2000s, the answer was largely negative. The plant’s enemies at home proved neither numerous nor damaging, none of them feeds on giant hogweed alone, and most of those that attack it in the Caucasus attack it in Europe too.⁵⁶⁴ Its success is now explained by other means.

The hypothesis

Every species introduced to a new region loses natural enemies, whether or not it goes on to become common. For that reason, critics argue, a shorter list of enemies abroad does not by itself explain why a particular invader thrives, and the hypothesis has often been accepted uncritically wherever an introduced species appears larger or more fecund than at home.²

Increased competitive ability

In 1995 Blossey and Nötzold added an evolutionary step. Plants in alien environments, they noted, tend to be more vigorous and taller and to produce more seed than in their native range. A plant with limited resources must divide them among maintenance, growth, storage, reproduction and defence; released from its enemies, it would be selected to spend less on defence and more on growth. They called this the evolution of increased competitive ability (EICA).⁷ Whether a plant has surplus photosynthate to spend on defensive chemistry at all depends, in a rival view they discussed, on the resources available to it.⁷

A third explanation, the “novel weapons” hypothesis, holds that some invaders carry chemicals to which the plants of the new range have no defence.⁸

In giant hogweed

Enemies at home and abroad

The European research project Giant Alien surveyed the insects on giant hogweed at 27 localities in nine countries over two seasons, and searched the literature for sixteen species of Heracleum. It recorded 358 insect species, 265 of them herbivores. About 162 herbivores were found on giant hogweed itself, of which 123 fed on many kinds of plant or had unknown habits and 39 fed on only one or a few. None is known to feed on giant hogweed alone.⁵

The share of specialists was higher in the Caucasus than in Europe, as the hypothesis predicts. But the insect orders were represented in much the same proportions in both ranges, and sap-sucking bugs were, if anything, more numerous in Europe.⁵ Two insects new to science were found in the Caucasus, the moth Agonopterix caucasiella and the fly Melanagromyza heracleana.⁵ In Switzerland, the native common hogweed and the giant each carried 34 species of plant-eating arthropod, and none had a major effect on the giant’s leaves and stems.⁹

In the native range, the insects boring inside the plant were dominated by three weevils, Lixus iridis, Nastus fausti and Otiorhynchus tatarchani, and by the fly Melanagromyza heracleana. None did serious damage, and mature plants proved quite tolerant of being eaten.⁶ Fungal pathogens were numerous, and one of them, Phloeospora heraclei, occurred at every site surveyed, causing leaf spot and die-back, especially in seedlings.¹⁰

Timing protects the plant as well. Once it flowers, its stored reserves are spent so quickly that no herbivore or pathogen has time to affect flowering.⁴growth outruns its critics

Defence

As summarised by the project, the plant’s costly defence, its glandular hairs, was weaker in the invaded range, as EICA predicts. Its cheaper chemical defence, the furanocoumarins, was stronger in Europe, where no damaging specialist attacks it, which is the opposite of the prediction.⁴ Both defences can be switched on by attack.¹¹

A related case is known from wild parsnip, another European member of the carrot family that became a weed in North America. Herbarium specimens covering 152 years show that American parsnips collected between 1850 and 1889 held less furanocoumarin than all those collected later, and less than European plants of the same period. The rise coincides with the accidental arrival of the parsnip webworm, the plant’s coevolved enemy. When the enemy caught up, the weed became more toxic, not less.¹²

Verdict

The Giant Alien authors read their survey as supporting one aspect of enemy release and of EICA.⁵ Others concluded that no enemy had a clearly greater impact in the Caucasus, so that the two hypotheses are “rather not relevant” to the species.¹³ A study of the plant’s age and reproduction in both ranges attributed its success to a more favourable climate in Europe and to the greater opportunities for dispersal in a densely settled continent.³ The project’s synthesis listed the negligible impact of natural enemies as one trait among many, in a plant with no weak link: a “master of all traits”.⁴

The search for biological control

In 2004, researchers argued that classical biological control, the release of a specific enemy from the native range, was the only sustainable option for large infestations, since the enemies found in Europe were generalist insects and fungi.¹⁰ The candidates were then tested one by one.

• The leaf fungus Phloeospora heraclei sporadically infected parsnip and coriander in the laboratory.¹⁰
• The weevil Lixus iridis did not feed or lay eggs on coriander, carrot or fennel, but laid few eggs even on the target.¹⁰
• Two damaging insects, the thrips Thrips vulgatissimus and the cabbage moth Mamestra brassicae, turned out to feed on many kinds of plant.¹⁰
• The root weevil Nastus fausti, offered no choice, fed on carrot, parsnip and celeriac with no loss of survival or fecundity, and its larvae could develop on their roots.¹⁴

None of the enemies from the Caucasus did enough damage at native densities, and none proved specific to giant hogweed or safe to release. All damaged other species of Heracleum, and most damaged parsnip.⁴ Introducing an exotic agent was judged impossible, and the project recommended studying a native European fungus as a mycoherbicide instead. European rules on plant protection products, it noted, all but excluded the release of an exotic fungus in any case.⁴ The project, which ran from 2002 to 2005, cost €2,934,773, of which the European Union contributed €1,721,459.¹¹no enemy of its own

Why the plant should have no specific enemies has been explained historically. During the ice ages the hogweeds of the Caucasus retreated into two small refugia, from which they expanded and mixed again in the warm periods between, producing the genus’s many forms and hybrids. Populations so small, the argument runs, would not have selected for enemies specialised on them.¹⁵

When the enemies catch up

Enemy release, if it occurs at all, may be temporary. At 24 Czech grasslands invaded for between 11 and 48 years, native species and productivity were at first reduced by giant hogweed but tended to recover after about 30 years, while the hogweed’s cover declined throughout. In a common-garden experiment, hogweed grown in soil taken from the sites invaded longest survived less well, grew less and competed worse: a negative feedback between plant and soil. The initial dominance of the invader, the authors concluded, can later be reversed.¹⁶

In the long run the stand prepares the conditions of its own decline.it produces its own gravediggers Invasions that rise and then collapse are known in the literature as boom-and-bust cycles.¹⁶¹⁷

See also

• Giant hogweed
• Heracleum
• Furanocoumarin
• Lag phase

Footnotes

1. Keane, R. M.; Crawley, M. J. (2002). “Exotic plant invasions and the enemy release hypothesis”. Trends in Ecology & Evolution. 17 (4): 164–170. doi:10.1016/S0169-5347(02)02499-0. ↩

2. Colautti, R. I.; Ricciardi, A.; Grigorovich, I. A.; MacIsaac, H. J. (2004). “Is invasion success explained by the enemy release hypothesis?”. Ecology Letters. 7 (8): 721–733. doi:10.1111/j.1461-0248.2004.00616.x. ↩ ↩²

3. Pergl, J.; Perglová, I.; Pyšek, P.; Dietz, H. (2006). “Population age structure and reproductive behavior of the monocarpic perennial Heracleum mantegazzianum (Apiaceae) in its native and invaded distribution ranges”. American Journal of Botany. 93 (7): 1018–1028. doi:10.3732/ajb.93.7.1018. ↩ ↩²

4. Pyšek, P.; Cock, M. J. W.; Nentwig, W.; Ravn, H. P. (2007). “Master of all traits: can we successfully fight giant hogweed?”. In Pyšek, P.; et al. (eds.). Ecology and Management of Giant Hogweed. Wallingford: CAB International. pp. 297–312. doi:10.1079/9781845932060.0297. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

5. Hansen, S. O.; Hattendorf, J.; Wittenberg, R.; Reznik, S. Ya.; Nielsen, C.; Ravn, H. P.; Nentwig, W. (2006). “Phytophagous insects of giant hogweed Heracleum mantegazzianum (Apiaceae) in invaded areas of Europe and in its native area of the Caucasus”. European Journal of Entomology. 103 (2): 387–395. doi:10.14411/eje.2006.052. ↩ ↩² ↩³ ↩⁴ ↩⁵

6. Hattendorf, J.; Hansen, S. O.; Reznik, S. Ya.; Nentwig, W. (2006). “Herbivore impact versus host size preference: endophagous insects on Heracleum mantegazzianum in its native range”. Environmental Entomology. 35 (4): 1013–1020. doi:10.1603/0046-225X-35.4.1013. ↩ ↩²

7. Blossey, B.; Nötzold, R. (1995). “Evolution of increased competitive ability in invasive nonindigenous plants: a hypothesis”. Journal of Ecology. 83 (5): 887–889. doi:10.2307/2261425. ↩ ↩²

8. Callaway, R. M.; Ridenour, W. M. (2004). “Novel weapons: invasive success and the evolution of increased competitive ability”. Frontiers in Ecology and the Environment. 2 (8): 436–443. doi:10.1890/1540-9295(2004)002[0436:NWISAT]2.0.CO;2. ↩

9. Bürki, C.; Nentwig, W. (1997). “Vergleichende Untersuchungen von herbivoren Insekten an Heracleum sphondylium und H. mantegazzianum in der Schweiz”. Entomologia Generalis. 22 (2): 147–155. doi:10.1127/entom.gen/22/1997/147. ↩

10. Seier, M. K.; Wittenberg, R.; Ellison, C. A.; Djeddour, D. H.; Evans, H. C. (2004). “Surveys for natural enemies of giant hogweed (Heracleum mantegazzianum) in the Caucasus region and assessment for their classical biological control potential in Europe”. In Cullen, J. M.; et al. (eds.). Proceedings of the XI International Symposium on Biological Control of Weeds. Canberra: CSIRO Entomology. pp. 149–154. ↩ ↩² ↩³ ↩⁴ ↩⁵

11. European Commission, CORDIS. “Giant hogweed (Heracleum mantegazzianum) a pernicious invasive weed: developing a sustainable strategy for alien invasive plant management in Europe”. Project EVK2-CT-2001-00128. Retrieved 26 September 2026. ↩ ↩²

12. Zangerl, A. R.; Berenbaum, M. R. (2005). “Increase in toxicity of an invasive weed after reassociation with its coevolved herbivore”. Proceedings of the National Academy of Sciences. 102 (43): 15529–15532. doi:10.1073/pnas.0507805102. ↩

13. Thiele, J. (2007). Patterns and processes of Heracleum mantegazzianum invasion into German cultural landscapes on the local, landscape and regional scale. Doctoral dissertation. Giessen: Justus-Liebig-Universität. ↩

14. Reznik, S. Ya.; Dolgovskaya, M. Yu.; Zaitzev, V. F.; Davidyan, G. E.; Nentwig, W. (2008). “Evaluation of Nastus fausti Reitter (Coleoptera: Curculionidae) for biological control of invasive giant hogweeds (Heracleum spp.)”. Entomological Review. 88 (6): 640–650. doi:10.1134/S001387380806002X. ↩

15. Cock, M. J. W.; Seier, M. K. (2007). “The scope for biological control of giant hogweed, Heracleum mantegazzianum”. In Pyšek, P.; et al. (eds.). Ecology and Management of Giant Hogweed. Wallingford: CAB International. pp. 255–271. doi:10.1079/9781845932060.0255. ↩

16. Dostál, P.; Müllerová, J.; Pyšek, P.; Pergl, J.; Klinerová, T. (2013). “The impact of an invasive plant changes over time”. Ecology Letters. 16 (10): 1277–1284. doi:10.1111/ele.12166. ↩ ↩²

17. Simberloff, D.; Gibbons, L. (2004). “Now you see them, now you don’t! – population crashes of established introduced species”. Biological Invasions. 6 (2): 161–172. doi:10.1023/B:BINV.0000022133.49752.46. ↩