When Wheat Drilling is About More Than Wheat

The Carrot Weevil

IT IS STILL AROUND

BY JUAN R. ANCISO

Adult Carrot Weevil

Acarrot weevil, Listronotus texanus (Stockton), has been the key pest affecting carrot, parsley, root parsley, dill and fennel production in South Texas for the past seventy years. The feeding damage by the larvae can kill the young plants of these crops but really this stage makes the insect difficult to control since it is protected from foliar insecticides. While this insect can be quite devastating to these five crops, it is processing carrots that can have the most economic impact by this insect. The presence of larvae and feeding damage in the root are of major concern to carrot processors because of stringent FDA quality control of insect parts in processed food that can lead to loads of carrots being rejected. In addition to loads being rejected, yields can be greatly reduced in processing carrots as well as fresh market carrots, parsley, root parsley, dill and fennel.

The cream-white larvae which are legless and relatively slow-moving have proven to be very difficult to control with insecticide sprays in the crown area and not controllable once in the roots.

Once these larvae become pupae in the soil it is impossible to attain any level of chemical control. The adult stage is targeted for chemical control and this chemical application is based primarily on sampling for eggs as an action threshold as an indication that adult carrot weevils have moved into the field. The eggs are difficult to sample due to their pinhead size (1mm long by 0.5 mm wide) and color but still possible for a well trained scout. The coloration and behavior of adults makes them impractical for monitoring either by traps or visual inspection.

Management programs from 1961 to 1989 were dependent on six insecticide applications on a calendar schedule specified in producer contracts. During the 1980’s the product often specified in producer contracts was Pydrin (fenvalerate). An iso-

Carrot weevil larvae and damage to carrot root

mer of Pydrin eventually became Asana (esfenvalerate) which was the product that was used in the early 1990’s for control. Studies were initiated to determine whether action thresholds were effective in comparison to calendar based sprays. Threshold applications of Asana (esfenvalerate) or Vydate (oxamyl) when first detecting carrot weevil eggs were as effective as calendar based sprays in research trials. This information was implemented into the Integrated Pest Management program for carrots and developed favorable producer acceptance because of its success in effectively controlling the carrot weevil and reducing insecticide applications from the six prescribed applications. However, as the program continued with action thresholds based on egg detections, fields that were treated only with Asana in comparison to Vydate continued to have problems in effectively controlling the carrot weevil. Collaborative research with the Agriculture Canada determined that a small parasitic beneficial wasp (Anaphes sp.) first found in unsprayed carrot plots in South Texas can give up to 70% control of the carrot weevil. This parasitic beneficial wasp is even smaller than the carrot weevil egg in which it lays its egg into the carrot weevil egg. In field situations this beneficial wasp is probably most negatively impacted by synthetic pyrethroids like Asana rather than by Vydate and may explain the success of Vydate in effectively controlling the

carrot weevil.

Carrot weevil eggs can fit in the ear of FDR Anaphes sp. a biological control agent (parasitic wasp) on a carrot weevil egg

Vydate has since been off and now back on the market for use in carrots and has had its list of labeled crops much reduced. Vydate is a very important chemical tool in the carrot IPM program in Texas where chemical control is complemented with the natural biological control for successful carrot weevil management. Texas carrot acreage has declined over the last 20 years with 7,100 acres in 1998; 2,400 acres in 2008; and 1,700 acres in 2018 which has mostly been located in South Texas. If the carrot industry is going to continue to be part of the vegetable landscape as well as those other crops affected by the carrot weevil, Integrated Pest Management programs as well as new chemical tools (Exirel-cyantraniliprole) will need to be tested, evaluated and labelled especially in the environment where chemical companies continue to merge with one another causing the splitting or divesting off effective chemical products needed on many minor specialty vegetable crops such as carrots.