Twig crotch feeding
Before elm bark beetles attack weakened or dying trees in order to breed,
they often perform twig crotch feeding in healthy trees Transmission of O. ulmis.l. to its host is thought to occur at this stage in the beetles' life cycle (see The Transmission Event).H. Rufipes bores tunnels in the phloem and feeds on the wood of limbs 5-10 cm (newly
emerged adults) and stems 5-25 cm (overwintering adults) in diameter.
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Photo 38: S. multistriatus feeding in the crotch of a small elm twig (Courtesy of
P. Svihra, University of California,Novato, CA, USA).
| Feeding activities by overwintering H. rufipes seem to occur in the lower 55 cm of the host tree {[670]}. The bark of
stems of 25 cm or more in diameter is too thick to allow penetration to the xylem {[482]}. The radial penetration of the xylem by H. rufipes rarely exceeds 1 mm. Although Scolytus beetles prefer rapidly growing twigs of one or more years old in the upper
periphery of the crown, feeding grooves can be cut in almost any young and sappy patch of roughened bark {[345],[381],[434]}.S. scolytus normally chooses twigs of at least two years old and rarely feeds on one
year old twigs. Infected twigs without wilting symptoms can also be used for feeding. The latter situation may result in internal contamination
of the beetles with O. ulmis.l.{[381]}. Beetles are found to aggregate in the top of the elm during feeding
{[353],[434]}.
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Feeding grooves vary in their structure and location. S. scolytus usually drills a vertical hole into the bark of the twig crotch or beneath
a side branch. In addition this beetle can cut extensive and even branched horizontal grooves (similar to maternal egg galleries) of 2-4 cm in length
by tunneling between bark and wood{[371],[381]}. Twig crotch feeding by S. multistriatus results in very small wounds (vertical holes) that may be as deep as 4
mm. Often these wounds are already healed before the elm shows DED symptoms. For feeding activities the small European elm bark beetle prefers
crotches of twigs with a diameter of 3-10 mm {[230],[381]}.
In early summer most feeding injuries occur at the junction formed by a
current years shoot and the previous years' growth. Beetle feeding activities later in summer will affect new shoots at the bases of leaf petioles
{[345]}. Fransen {[381]} reported that in periods with cold weather conditions, twig feeding is replaced by feeding on larger branches and stems of weakened and dying trees. Von Keyserlingk {[347]} observed that S. scolytus orients reliably to small pieces of elm twigs ignoring the twigs of non-hostplants
like beech, oak, alder and lime. Attractants (attraction pheromones) released by attacking beetles can override attraction to twig crotches and non-infested
diseased elms {[345]}. Scolytus beetles can stay in the feeding grooves for up to 13 days {[381]}. H. rufipes is reported to feed for a few weeks {[5]}.
The function of twig crotch feeding is not completely understood. It may
be associated with copulation, restoration of the beetles reserves (food and/or water) during hibernation or function
as a prerequisite for sexual maturation (e.g. for S. pygmaeus and S. mali) {[234],[345],[353],[363],[381]}. Another possibility is that the beetle
uses the feeding cavity in the crotch as protection, waiting for favourable weather conditions before selecting a breeding site {[353]}. In contrast
to Fransen {[381]}, Sinclair et al. {[230]} reported that S. multistriatus and S. scolytus do not necessarily need to feed first on healthy elm before attacking weakened
trees in order to breed. Recently, White {[671]} showed that twig crotch feeding in S. multistriatus serves as pre-vittellogenic feeding required for female beetles to complete
oogenesis.
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