Friendly fighter conquers foe

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Micropli­tis demoli­tor is just one of many friend­ly fight­ers that bat­tle to con­tain num­bers of one of our most impor­tant pests, the corn ear­worm, Heli­cov­er­pa armigera.

Corn ear­worm on grain sorghum is mak­ing its pres­ence felt and many crops are being sprayed with Heli­cov­er­pa nucle­opoly­he­drovirus (NPV) to con­trol above-thresh­old infes­ta­tions of cater­pil­lars.

The cur­rent high val­ue of grain sorghum (over $250 per tonne) means that it is eco­nom­ic to con­trol cater­pil­lars at low­er num­bers (den­si­ty) than grow­ers may have sprayed pre­vi­ous­ly when grain val­ue was low­er.

Low cater­pil­lar num­bers is a per­fect sit­u­a­tion for Micropli­tis to chip in a help­ing hand. It is not uncom­mon to find 30–40% of small cater­pil­lars on grain sorghum par­a­sitised by Micropli­tis. In many cas­es, this lev­el of par­a­sitism may be suf­fi­cient to sway a deci­sion to not spray.

What is Micropli­tis?
Micropli­tis is a small native wasp that lays it eggs in (par­a­sitis­es) small heli­cov­er­pa cater­pil­lars. The life cycle from egg to adult takes about 12 days. This is made up of 7 days from egg lay­ing to form­ing a pupa, and then 5 days for pupal devel­op­ment.

Adult Micropli­tis are small black-brown wasps. They are often seen fly­ing slow­ly above the crop canopy in search of cater­pil­lars (hosts). A female wasp will par­a­sitise as many as 70 heli­cov­er­pa cater­pil­lars. The par­a­site devel­ops inside the host cater­pil­lar. When ful­ly devel­oped, the Micropli­tis lar­va chews a hole in the side of the cater­pil­lar and spins a fawn-coloured cocoon around itself and pupates. The cater­pil­lar that was par­a­sitised may still be alive, but it will soon die.

Clues to iden­ti­fy Micropli­tis activ­i­ty include

  • Adult wasps for­ag­ing on sorghum heads
  • Split test of cater­pil­lars to reveal inter­nal par­a­sites
  • Dis­tinc­tive fawn cocoons next to dead or dying cater­pil­lars

Iden­ti­fy­ing par­a­sitised cater­pil­lars
In the field, par­a­sitised cater­pil­lars can be iden­ti­fied by per­form­ing a sim­ple split test. Par­a­sitised cater­pil­lars will only grow to about 15 mm in length, so cater­pil­lars small­er than this are poten­tial­ly Micropli­tis hosts. Hold a cater­pil­lar across a fore­fin­ger with one thumb and fore­fin­ger on the rear end of the cater­pil­lar, and with the oth­er thumb on the head. Gen­tly stretch the cater­pil­lar until the skin rup­tures. A Micropli­tis lar­va devel­op­ing with­in the cater­pil­lar looks like a white mag­got up to 4 mm long.

Inter­ac­tions between Micropli­tis and NPV
Cater­pil­lars infect­ed with NPV with­in 3 days of par­a­siti­sa­tion by Micropli­tis will die from NPV. The imma­ture Micropli­tis will also die because of the death of its host.

When NPV is applied to con­trol corn ear­worm, it is not unusu­al for some par­a­sitised cater­pil­lars to sur­vive the treat­ment. Cater­pil­lars par­a­sitised more than 3 days pri­or to the NPV treat­ment will pro­duce healthy Micropli­tis. Par­a­sitised cater­pil­lars feed less and may not ingest NPV.

In shak­ing sorghum heads to make post-treat­ment assess­ments, par­a­sitised lar­vae may be dis­lodged free of the pupal cocoon attached to them. Care­ful inspec­tion of these cater­pil­lars may reveal a hole in the side of some of these cater­pil­lars, indi­cat­ing pri­or par­a­siti­sa­tion. These lar­vae will even­tu­al­ly die.

Micropli­tis is an impor­tant nat­ur­al ene­my of the corn ear­worm and they need to be con­sid­ered when mak­ing deci­sions about when to man­age corn ear­worm.

For more infor­ma­tion on Micropli­tis, vis­it the QDAFF web­site.