About those fall web­worms

Home - Santa Fe Real Estate Guide - - THEMASTERGARDENERS - TERRY MCGUIRE

The spring gar­den­ing sea­son in New Mex­ico of­ten starts with the emer­gence of the western tent cater­pil­lar ( Mala­co­soma cal­i­for­nicum). Th­ese cater­pil­lars build messy silk nests and eat young leaves on trees. The New Mex­ico gar­den­ing sea­son ends with the ap­pear­ance of fall web­worms ( Hyphan­tria cunea), cater­pil­lars that also build messy silk nests but eat the ma­ture leaves on trees. The adult web­wor­m­moth is white and may have black or brown spots on the forewings and red­dish-or­ange patches on its front legs. Their wing span is about 1¼ to 1½ inches. In most of New Mex­ico, the moths prob­a­bly have only one re­pro­duc­tive cy­cle per year. Adults emerge from their pupa case in late June or early July. They mate and the fe­male lays an egg mass of 300 to 900 eggs on the un­der­sides of leaves. The eggs hatch in 7 to 10 days.

The newly hatched cater­pil­lars build a small silk nest that en­closes leaves at the end of branches. Ini­tially they “skele­tonize” the leaves, eat­ing the tis­sue but not the veins. Larger cater­pil­lars eat en­tire leaves. The web­worms en­large their tent as they grow, even­tu­ally en­clos­ing en­tire branches. The cater­pil­lars have black or red heads. Black-headed cater­pil­lars have pale yel­low or pale green bod­ies, while red-headed cater­pil­lars have darker, yel­low-tan bod­ies. The lar­vae start out quite small and grow to a fi­nal length of 1-1¼ inches. The nests be­come quite messy as they fill up with shed skins, in­sect drop­pings, and leaf frag­ments. Cater­pil­lars con­tinue to feed into Sep­tem­ber. Even­tu­ally, the fully grown cater­pil­lars leave the nest and form a pu­pal case in­side a silk co­coon in a pro­tected place in the soil or in leaf lit­ter, or on the sides of trees, rocks, or build­ings.

The fall web­worm can feed on hun­dreds of dif­fer­ent de­cid­u­ous trees and shrubs. It has been recorded on 636 dif­fer­ent species, in­clud­ing cot­ton­woods, wal­nut, alder, poplars, many fruit and nut trees, and shrubs. In gen­eral, they will eat any tree in New Mex­ico ex­cept pines, firs, spruces and ju­nipers.

Al­though the nests are ex­tremely unattrac­tive, the cater­pil­lars cause lit­tle dam­age to the tree. The ma­ture leaves that they eat are no longer nec­es­sary for the tree’s food re­serves. Web­worm in­fes­ta­tions tend to de­cease over time due to par­a­sites, preda­tors and bac­te­rial and vi­ral dis­eases. The cater­pil­lars are an im­por­tant source of food for sev­eral species of song­birds, bats, and small mam­mals.

Nests can be re­moved me­chan­i­cally. Pull down the webs with a long stick or rake and de­stroy the web­worms by drown­ing them in spray water or seal­ing themin a garbage bag. Tear­ing a hole in the nest will al­low nat­u­ral preda­tors to get at the web­worms. Cut­ting off the branches with the nest may dam­age the tree. Do not use fire to de­stroy the nest. This not only dam­ages the tree but could start a wild­fire. It is pos­si­ble to use pes­ti­cides such as Sevin or some Bt for­mu­la­tions; how­ever, th­ese pes­ti­cides are most ef­fec­tive when ap­plied in the early sum­mer be­fore the nests get large. If you de­cide to use pes­ti­cides, it is es­sen­tial to make sure that they are reg­is­tered for fall web­worm con­trol and to fol­lowla­bel in­struc­tions.

Terry McGuire was pro­fes­sor of ge­net­ics at Rut­gers Uni­ver­sity for 36 years. He was also a Se­nior Fel­low of theNa­tional Cen­ter for Sci­ence and Civic En­gage­ment, help­ing ed­u­ca­tors con­nect sci­ence to civic is­sues. He moved to Santa Fe in 2014. He is a Mas­ter Gar­dener and will soon be a Mas­ter Com­poster.



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