Defining the window of opportunity for feeding initiation by second-instar spruce budworm larvae

Abstract

Establishing feeding sites is critical for the survival of neonate Lepidoptera larvae. Rapid foliar quality changes during leaf expansion create a narrow window of opportunity for establishment of early-spring feeders. We examined the effect of phenological synchrony between black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.) and balsam fir (Abies balsamea (L.) Mill.) budbreak and spruce budworm (Choristoneura fumiferana (Clemens)) emergence on the feeding behaviour of young larvae and on overall larval growth and survival under laboratory conditions. We correlated these variables with bud development and foliar toughness during the growing season. Our results show that early-emerging second-instar larvae were unable to feed on either black spruce or balsam fir buds; budworm on balsam fir mined old foliage and exhibited good survival and performance, but those on black spruce remained on the foliar surface and suffered high mortality and low growth. In the second later-emerging cohort, bud feeding gradually increased on black spruce whereas it was already the predominant behaviour on balsam fir, and no differences in performance were observed between host species. Thus, black spruce budbreak constitutes a strict window of opportunity, since larvae are often unable to mine the old foliage. Our results suggest that mechanical toughness could be the obstacle preventing young larvae from mining old black spruce needles. Our findings confirm the importance of second-instar ecology in spruce budworm, suggesting that, if climate warming eventually results in an improvement in phenological synchrony between spruce budworm and black spruce, larval survival may increase.

Warming counteracts defoliation‐induced mismatch by increasing herbivore‐plant phenological synchrony

“…[Mill.]). Balsam fir is the main host of spruce budworm due to the tight synchrony with larval development via food quality, that is, needle characteristics (Despland et al, 2011;Fuentealba, Sagne, Pureswaran, Bauce, & Despland, 2018;Maclean, 1984). The timing of budburst is critical for young larvae because the suitability of foliage as food declines quickly after budburst (Quiring & McKinnon, 1999).…”

Phenological synchrony between eastern spruce budworm and its host trees increases with warmer temperatures in the boreal forest

“…Emerging too late in relation to budburst also reduces fitness as lignified foliage is difficult for young larvae to feed on (Lawrence et al, ). Recent studies that manipulated the availability of new buds to second instar larvae showed that phenological asynchrony with both balsam fir and black spruce has a negative impact on spruce budworm performance (Fuentealba, Pureswaran, Bauce, & Despland, ; Fuentealba, Sagne, Pureswaran, Bauce, & Despland, ).…”

An alternative hypothesis explains outbreaks of conifer‐feeding budworms of the genus Choristoneura (Lepidoptera: Tortricidae) in Canada

“…Having hibernated mostly in the crown of the trees where the eggs were laid, the following spring they leave their hibernacula and commence feeding on old needles (McGugan, ), sometimes for weeks (Nealis, ), mining them selectively to eat only the mesophyll that contains the highest level of nitrogen, thus providing them with “… sustenance in the midst of privation.” (Tier & Mattson, ), but this sustenance may still not be sufficient to enable more than a few of these larvae to survive, especially when they emerged weeks before bud‐burst. And just how important the ability to access these old needles can be to the success of these neonate larvae is illustrated by their failure to survive on host species whose needles they are unable to feed on (Fuentealba, Sagne, Pureswaran, Bauce, & Despland, ). Even so it may need the addition of weather‐induced stress of these needles to hasten the rate of breakdown of protein to soluble amino acids.…”

“…(Tier & Mattson, 1997), but this sustenance may still not be sufficient to enable more than a few of these larvae to survive, especially when they emerged weeks before bud-burst. And just how important the ability to access these old needles can be to the success of these neonate larvae is illustrated by their failure to survive on host species whose needles they are unable to feed on (Fuentealba, Sagne, Pureswaran, Bauce, & Despland, 2018). Even so it may need the addition of weather-induced stress of these needles to hasten the rate of breakdown of protein to soluble amino acids.…”